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AS 


HARPER'S   SCIENTIFIC  MEMOIRS 

EDITED  BY 

J.  S.  AMES,  Ph.D. 

PROFESSOR    OF   PHYSICS    IN    JOHNS   HOPKINS    UNIVERSITY 


III. 

RONTGEX    RAYS 


Digitized  by  the  Internet  Archive 

in  2011  with  funding  from 

Research  Library,  The  Getty  Research  Institute 


http://www.archive.org/details/rontgenraysmemoirOOront 


R  O  NTGEN     RAYS 


MEMOIRS  BY  ROXTGEK,  STOKES 
AXD   J.  J.  THOMSON 


TRANSLATED  AND  EDITED  BY 

GEORGE  F.  BARKER,  LL.D. 

PROFESSOR    OF   PHYSICS    IN    THE    UNIVERSITY    OF   PENNSYLVANIA 


NEW  YORK  AND  LONDON 

HARPER     &     BROTHERS     PUBLISHERS 

1899 


HARPER'S    SCIENTIFIC   MEMOIRS. 

EDITED    BY 

J.    S.  AMES,  Ph.D., 

PROFK880B   OF   PHYSIOS    IN    JOHNS    UOPKIN8  UNIVEUSITY. 


.voir  READY: 

THE  FREE  EXPANSION  OF  GASES.  Memoirs 
by  Gay-Lussac,  Joule,  aud  Joule  an<i  Thomson. 
Editoi,  I'rof,  J.  S.  Amics,  Ph.D.,  Johns  Hopkins 
Uuiveri^ity.     75  cents. 

PRISMATIC  AND  DIFFRACTION  SPECTRA. 
Memoirs  by  Joseph  von  Fraunhofer.  Editor,  Prof. 
J.  S.  A.MKS,  Ph.D.,  Johns  Hopkins  University. 
00  cents. 

RONTGEN  RAYS.  Memoirs  by  Ront^en,  Stokes, 
and  J.  J.  Thomson.  Editor,  Prof.  Gkougk  F, 
Barkkk,  University  of  Pennsylvania. 

/.Y  PREPARATION: 
THE    MODERN   THEORY   OF    SOLUTION.      Me- 
moirs by  Pfeffer,  Van't.  Hoff,  Arrhenius,  and  Raoult. 
Editor,  Dr.  H.  C.  Jonks,  Johns  Hopkins  University. 

ON  THE  LAWS  OF  GASES.  Memoirs  by  Boyle, 
Amatzat,  Gay-Lussac.  Editor,  Prof.  Caul  Baui;s. 
Brown  L'ni  versify. 

THE  SECOND  LAW  OF  THERMODYNAMICS. 
Memoirs  by  Caruot,  Clausins,  and  Thomson.  Editor, 
Prof.  W.  F.  Magik,  Princeton  I'niversity. 

ON  THE  PROPERTIES  OF  lON.s.  Memoirs  by 
Kohhatisch  and  Ilittorf.  Editor,  Dr.  H.  M.  Good- 
win, Massachusetts  Institute  of  Technology. 

THE  FARADAY  AND  ZEEMAN  EFFECT.  Me- 
moirs t)y  Faradav,  Kerr,  and  Zeeman.  Editor,  Dr. 
E.  P.  Lkwis,  University  of  California. 

WAVE-THEORY  OF  LIGHT.  Memoirs  by  Young 
and  Fresnel.  Editor,  Prof.  IIenby  Ciikw,  North- 
western University. 

NEWTON'S  LAW  OF  GRAVITATION.  Editor, 
Prof.  A.  S.  Maokknzik,  Bryn  Mawr  College. 

NEW    YORK    AND    LONDON: 
HARPER  &  BROTHERS,  PUBLISHERS. 


Copyright,  1S98,  by  Haupf.u  &  Hkothkrs. 

Atl  rights  rftirvtii. 


THE  GETTY  Ctf.ith 
LIBRARY 


PREFACE 


The  new  kind  of  radiation  known  as  X-rays,  or  Eontgen 
rays,  from  the  name  of  their  discoverer,  were  first  observed  and 
studied  by  Professor  W.  C.  Rontgen,  of  the  University  of  Wilrz- 
burg,  in  1895,  and  the  announcement  of  their  discovery  was 
made  in  a  paper  which  appeared  that  year,  and  which  is  re- 
printed in  this  volume.  As  was  noticed  later  these  radiations 
had  been  previously  detected  and  some  of  their  properties 
noted  by  other  observers,  notably  Professor  Lenard  ;  but  it  is 
to  Rontgen  that  we  owe  the  first  systematic  study  of  the  meth- 
ods of  production  and  of  the  remarkable  properties  of  these 
rays.  Nearly  all  the  general  properties,  both  positive  and  neg- 
ative, were  investigated  by  Rontgen  and  carefully  stated. 
These  results  are  contained  in  the  first  three  pages  of  this 
volume. 

The  most  important  experiments,  however,  and  those  which 
have  led  to  the  most  important  conclusions,  were  made  by  Pro- 
fessor J.  J.  Thomson,  of  Cambridge.  They  proved  the  fact 
that  a  dielectric  traversed  by  these  radiations  became  a  con- 
ductor, or,  in  other  words,  was  ionized.  This  discovery  in  the 
hands  of  Professor  Thomson  and  his  students  has  led  to  a 
series  of  most  interesting  and  important  researches,  all  bearing 
upon  the  intimate  connection  between  matter  and  electricity. 

Many  hypotheses  have  been  advanced  to  accouut  for  the  pe- 
culiar properties  of  the  X-rays.  Rontgen  himself  at  first  was 
favorably  inclined  to  the  idea  that  they  were  waves  due  to  lon- 
gitudinal vibrations  in  the  ether,  but  later  he  was  convinced 
that  they  were  essentially  identical  with  light  waves — that  is, 


PREFACE 

with  transverse  waves  in  the  ether.  There  were  grave  obsta- 
cles, from  many  stand-points,  to  either  of  these  theories,  and 
the  first  suggestion  which  seemed  to  offer  a  satisfactory  expLa- 
nation  of  all  the  properties  of  the  rays  came  when,  instead  of 
waves,  the  idea  of  pulses  in  the  ether  was  introduced.  This 
idea  in  its  simplicity  is  that  the  cathode  rays  being  negative- 
ly charged  and  travelling  with  great  velocity,  give  rise  to  in- 
tensely sudden  disturbances  in  the  ether  when  their  motions 
are  stopped  by  reaching  a  solid  obstacle.  These  disturbances 
are  of  the  nature  of  irregular  pulses,  and  their  properties  are 
quite  different  from  those  of  regular  trains  of  waves. 

This  idea  of  accounting  for  Rontgen  rays  by  the  theory  of 
pulses  occurred  almost  simultaneously  to  Sir  George  Gabriel 
Stokes,  to  Professor  J.  J.  Thomson,  and  to  Professor  Lehmann, 
of  Karlsruhe.  Stokes's  paper,  in  which  he  explains  his  theory, 
is  reproduced  in  full  in  this  volume,  as  are  also  the  essential 
portions  of  Professor  Thomson's  article. 


GENERAL    CONTEIS^TS 


PAGE 

Preface v 

A  New  Kind  of  Rays.     First  Communication.     By  W.  C.  Rontgen. ...  3 

Second  Communication.  By  W.  C.  Rontgen.  .  13 
Furtlier  Observations  on  the  Properties  of  the  X-Rays.     Bj^  W.  C. 

Rontgen 21 

Biographical  Sketch  of  Rontgen 40 

On    the    Nature    of    the    Rontgen    Rays.      ( The   Wilde    Lecture. ) 

By  Sir  G.  G.  Stokes,  Bart 43 

Biographical  Sketch  of  Stokes 66 

A  Theory  of  the  Connection  between  Cathode  and   Rontgen  Rays. 

By  J.  J.  Thomson 69 

Biographical  Sketch  of  Thomson 73 

Bibliography 74 

Index 75 


ON   A  NEW  KIND  OF  RAYS 


BY 


TWO    CO  21 M  UNI  CATIONS 

Sitzungsheridite  der  Wurzburger  PJiysikaUschen-^IediciniscJien  Gesellschoft, 
1895 — Wiedemann,  Annalen  der  Physih  iind  Chemie,  64,  1898 


CONTENTS 


FraST  COMMUNICATION 

PAGE 

Discovei^  of  X-Rays 3 

' '  Transparency  "  of  Substances 4 

Pi'operiies  Investigated. 

Fluorescence 6 

PJwtograpMc  Action 6 

Refraction  and  Reflection 7 

Variation  of  Intensity  icith  Distance 10 

Magnetic  Deflection 10 

Point  of  Emission  of  Rays 11 

Propagation  in  Straight  Lines 11 

Interference , 12 

Polarization 12 

Possible  Explanations 12 

Longitudinal  Vibrations  in  the  Ether 13 


SECOND  COMMUNICATION 

Electrical  Properties  of  X-Rays 13 

Discharge  of  Electnfied  Bodies 14 

Action  on  a  Dielectric 14 

Duration  of  Effect  on  Air  and  Otiier  Gases 15 

Centre  of  Emission  of  the  Rays 17 

Eff^ect  of  Different  Metals 18 


ON  A  NEW  KIND   OF  RAYS 

BY 

T\'.  C.  ROXTGEN 


FIRST    CO  MM  UXI CATIO  N 

1.  If  the  discharge  of  a  fairly  large  iuduction-coil  be  made  to 
pass  through  a  Hittorf  vacuum-tube,  or  through  a  Lenard  tube, 
a  Crookes  tube,  or  other  simiUir  apparatus,  which  has  been  suf- 
ficiently exhausted,  the  tube  being  covered  with  thin,  black 
card-board  which  fits  it  with  tolerable  closeness,  and  if  the 
whole  apparatus  be  placed  in  a  completely  darkened  room,  there 
is  observed  at  each  discharge  a  bright  illumination  of  a  pa- 
per screen  covered  with  barium  platino-cyanide,  placed  in  the 
vicinity  of  the  induction-coil,  the  fluorescence  thus  produced 
being  entirely  independent  of  the  fact  whether  the  coated  or 
the  plain  surface  is  turned  towards  the  discharge-tube.  This 
fluorescence  is  visible  even  when  the  paper  screen  is  at  a  dis- 
tance of  two  metres  from  the  apparatus. 

It  is  easy  to  prove  that  the  cause  of  the  fluorescence  proceeds 
from  the  discharge-apparatus,  and  not  from  any  other  point  in 
the  conducting  circuit. 

2.  The  most  striking  feature  of  this  phenomenon  is  the  fact 
that  an  active  agent  here  passes  through  a  black  card-board  en- 
velope, which  is  opaque  to  the  visible  and  the  ultra-violet  rays 
of  the  sun  or  of  the  electric  arc ;  an  agent,  too,  which  has  the 
power  of  producing  active  fluorescence.  Hence  we  may  first 
investigate  the  question  whether  other  bodies  also  possess  this 
property. 

We  soon  discover  that  all  bodies  are  transnarent  to  this  asrent, 

3 


MEMOIRS    ON 

though  in  very  different  degrees.  I  proceed  to  give  a  few  ex- 
amples :  Paper  is  very  transparent ;  *  behind  a  bound  book  of 
about  one  thousand  pages  I  saw  the  fluorescent  screen  light 
up  brightly,  the  printers'  ink  offering  scarcely  a  noticeable 
hinderance.  In  the  same  way  the  fluorescence  appeared  behind 
a  double  pack  of  cards ;  a  single  card  held  between  the  ap- 
paratus and  the  screen  being  almost  unnoticeable  to  the  eye. 
A  single  sheet  of  tin-foil  is  also  scarcely  perceptible  ;  it  is  only 
after  several  layers  have  been  placed  over  one  another  that 
their  shadow  is  distinctly  seen  on  the  screen.  Thick  blocks 
of  wood  are  also  transparent,  pine  boards  two  or  three  centi- 
metres thick  absorbing  only  slightly.  A  plate  of  aluminium 
about  fifteen  millimetres  thick,  though  it  enfeebled  the  action 
seriously,  did  not  cause  the  fluorescence  to  disappear  entirely. 
Sheets  of  hard  rubber  several  centimetres  thick  still  permit  the 
rays  to  pass  through  them.f  Glass  plates  of  equal  thickness 
behave  quite  differently,  according  as  they  contain  lead  (flint- 
glass)  or  not  ;  the  former  are  much  less  transparent  than  the 
latter.  If  the  hand  be  held  between  the  discharge-tube  and 
the  screen,  the  darker  shadow  of  the  bones  is  seen  within  the 
slightly  dark  shadow-image  of  the  hand  itself.  Water,  carbon 
disulphide,  and  various  other  liquids,  when  they  are  examined 
in  mica  vessels,  seem  also  to  be  transparent.  That  hydrogen  is 
to  any  considerable  degree  more  transparent  than  air  I  have 
not  been  able  to  discover.  Behind  plates  of  copper,  silver, 
lead,  gold,  and  platinum  the  fluorescence  may  still  be  recog- 
nized, though  only  if  the  thickness  of  the  plates  is  not  too 
great.  Platinum  of  a  thickness  of  0.2  millimetre  is  still  trans- 
parent ;  the  silver  and  copper  plates  may  even  be  thicker. 
Lead  of  a  thickness  of  1.5  millimetres  is  practically  opaque  ; 
and  on  account  of  this  property  this  metal  is  frequently  most 

*  By  "transparency"  of  a  bod}^  I  denote  the  relative  brightness  of  a 
fluorescent  screen  placed  close  behind  the  body,  referred  to  tiie  brightness 
which  the  screen  shows  under  the  same  circumstances,  though  without  the 
interposition  of  the  body. 

\  For  brevity's  sake  I  shall  use  the  expression  "  rays";  and  to  distinguish 
them  from  others  of  this  name  I  shall  call  them  "  X-rays."    (See  p.  11.) 

4 


ROXTGEN    RAYS 

useful.  A  rod  of  wood  with  a  square  cross -section  (20  x  20 
millimetres),  one  of  whose  sides  is  painted  white  with  lead  paint, 
behaves  differently  according  as  to  how  it  is  held  between  the 
ajDparatus  and  the  screen.  It  is  almost  entirely  without  action 
when  the  X-rays  pass  through  it  parallel  to  the  painted  side  ; 
whereas  the  stick  throws  a  dark  shadow  when  the  rays  are 
made  to  traverse  it  perpendicular  to  the  painted  side.  In  a 
series  similar  to  that  of  the  metals  themselves  their  salts  can 
be  arranged  with  reference  to  their  transparency,  either  in  the 
solid  form  or  in  solution. 

3.  The  experimental  results  which  have  now  been  given, 
as  well  as  others,  lead  to  the  conclusion  that  the  transpar- 
ency of  different  substances,  assumed  to  be  of  equal  thick- 
ness, is  essentially  conditioned  upon  their  density  :  no  other 
property  makes  itself  felt  like  this,  certainly  to  so  high  a  de- 
gree. 

The  following  experiments  show,  however,  that  the  density 
is  not  the  only  cause  acting.  I  have  examined,  with  reference 
to  their  trans2:)arency,  plates  of  glass,  aluminium,  calcite,  and 
quartz,  of  nearly  the  same  thickness  ;  and  while  these  sub- 
stances are  almost  equal  in  density,  yet  it  was  quite  evident 
that  the  calcite  was  sensibly  less  transparent  than  the  other 
substances,  which  appeared  almost  exactly  alike.  Xo  particu- 
larly strong  fluorescence  (see  p.  6  below)  of  calcite,  especially 
by  comparison  with  glass,  has  been  noticed. 

4.  All  substances  with  increase  in  thickness  become  less 
transparent.  In  order  to  find  a  possible  relation  between  trans- 
parency and  thickness,  I  have  made  photographs  (see  p.  6  be- 
low) in  which  portions  of  the  photographic  plate  were  covered 
with  layers  of  tin-foil,  varying  in  the  number  of  sheets  super- 
posed. Photometric  measurements  of  these  will  be  made  when 
I  am  in  possession  of  a  suitable  photometer. 

5.  Sheets  of  platinum,  lead,  zinc,  and  aluminium  were  rolled 
of  such  thickness  that  all  appeared  nearly  equally  transj^arent. 
The  following  table  contains  the  absolute  thickness  of  these 
sheets  measured  in  millimetres,  the  relative  thickness  referred 
to  that  of  the  platinum  sheet,  and  their  densities : 

5 


MEMOIRS    ON 

Thickness                         Rklative  Thickness 

Densiti 

Pt  0.018  mm.                       1 

21.5 

Pb  0.05      ''                         3 

11.3 

ZnO.lO      "                          6 

7.1 

Al  3.5        ''                      200 

2.6 

We  may  conclude  from  these  values  that  different  metals 
possess  transparencies  which  are  by  no  means  equal,  even  when 
the  product  of  thickness  and  density  are  the  same.  The  trans- 
parency increases  much  more  rapidly  than  this  product  de- 
creases. 

6.  The  fluorescence  of  barium  platino- cyanide  is  not  the 
only  recognizable  effect  of  the  X-rays.  It  should  be  mentioned 
that  other  bodies  also  fluoresce  ;  such,  for  instance,  as  the  phos- 
phorescent calcium  compounds,  then  uranium  glass,  ordinary 
glass,  calcite,  rock-salt,  and  so  on. 

Of  special  signiflcance  in  many  respects  is  the  fact  that 
photographic  dry  plates  are  sensitive  to  the  X-rays.  We  are, 
therefore,  in  a  condition  to  determine  more  definitely  many 
phenomena,  and  so  the  more  easily  to  avoid  deception ;  wher- 
ever it  has  been  possible,  therefore,  I  have  controlled,  by  means 
of  photography,  every  important  observation  which  I  have  made 
with  the  eye  by  means  of  the  fluorescent  screen. 

In  these  experiments  the  property  of  the  rays  to  pass  almost 
unhindered  through  thin  sheets  of  wood,  paper,  and  tin-foil  is 
most  important.  The  photographic  impressions  can  be  ob- 
tained in  a  non-darkened  room  with  the  j)hotograpliic  plates 
either  in  the  holders  or  wrapped  up  in  paper.  On  the  other 
hand,  from  this  property  it  results  as  a  consequence  that  un- 
developed plates  cannot  be  left  for  a  long  time  in  the  neighbor- 
hood of  the  discharge-tube,  if  they  are  protected  merely  by  the 
usual  covering  of  pasteboard  and  paper. 

It  appears  questionable,  however,  whether  the  chemical  ac- 
tion on  the  silver  salts  of  the  photographic  plates  is  directly 
caused  by  the  X-rays.  It  is  possible  that  this  action  proceeds 
from  the  fluorescent  light  which,  as  noted  above,  is  produced 

6 


RONTGEN    RAYS 

in   the  glass  plate  itself  or  perhaps  in  the  layer  of  gelatin. 
"Films  "  can  be  used  just  as  well  as  glass  plates. 

I  have  not  yet  been  able  to  prove  experimentally  that  the 
X-rays  are  able  also  to  produce  a  heating  action;  yet  we  may 
well  assume  that  this  effect  is  present,  since  the  capability  of 
the  X-rays  to  be  transformed  is  proved  by  means  of  the  ob- 
served fluoresence  phenomena.  It  is  certain,  therefore,  that 
all  the  X-rays  which  fall  upon  a  substance  do  not  leave  it  again 
as  such. 

The  retina  of  the  eye  is  not  sensitive  to  these  rays.  Even  if 
the  eye  is  brought  close  to  the  discharge-tube,  it  observes  noth- 
ing, although,  as  experiment  has  proved,  the  media  contained 
in  the  eye  must  be  sufficiently  transparent  to  transmit  the 
rays. 

7.  After  I  had  recognized  the  transparency  of  various  sub- 
stances of  relatively  considerable  thickness,  I  hastened  to  see 
how  the  X-rays  behaved  on  passing  through  a  prism,  and  to 
find  whether  they  were  thereby  deviated  or  not. 

Experiments  with  water  and  with  carbon  disulphide  enclosed 
in  mica  prisms  of  about  30°  refracting  angle  showed  no  devia- 
tion, either  with  the  fluorescent  screen  or  on  the  photographic 
plate.  For  purposes  of  comparison  the  deviation  of  rays  of 
ordinary  light  under  the  same  conditions  was  observed;  and  it 
was  noted  that  in  this  case  the  deviated  images  fell  on  the 
plate  about  10  or  20  millimetres  distant  from  the  direct  image. 
By  means  of  prisms  made  of  hard  rubber  and  of  aluminium, 
also  of  about  30°  refracting  angle,  I  have  obtained  images 
on  the  photographic  plate  in  which  some  small  deviation  may 
perhaps  be  recognized.  However,  the  fact  is  quite  uncertain; 
the  deviation,  if  it  does  exist,  being  so  small  that  in  any  case 
the  refractive  index  of  the  X-rays  in  the  substances  named 
cannot  be  more  than  1.05  at  the  most.  With  a  fluorescent 
screen  I  was  also  unable  to  observe  any  deviation. 

Up  to  the  present  time  experiments  with  prisms  of  denser 
metals  have  given  no  definite  results,  owing  to  their  feeble 
transparency  and  the  consequently  diminished  intensity  of  the 
transmitted  rays. 

7 


MEMOIRS    ON 

"Witli  reference  to  the  general  conditions  here  involved  on 
the  one  hand,  and  on  the  other  to  the  im2:)ortance  of  the  ques- 
tion whether  the  X-rays  can  be  refracted  or  not  on  i:)assing 
from  one  medium  into  another,  it  is  most  fortunate  that  this 
subject  may  be  investigated  in  still  another  way  than  with 
the  aid  of  prisms.  Finely  divided  bodies  in  sufficiently  thick 
layers  scatter  the  incident  light  and  allow  only  a  little  of  it 
to  pass,  owing  to  reflection  and  refraction ;  so  that  if  powders 
are  as  transparent  to  X-rays  as  the  same  substances  are  in  mass 
— equal  amounts  of  material  being  presupposed — it  follows  at 
once  that  neither  refraction  nor  regular  reflection  takes  place 
to  any  sensible  degree.  Experiments  w^ere  tried  with  finely 
powdered  rock-salt,  with  fine  electrolytic  silver-powder,  and 
with  zinc-dust,  such  as  is  used  in  chemical  investigations.  In 
all  these  cases  no  difference  was  detected  between  the  trans- 
parency of  the  powder  and  that  of  the  substance  in  mass, 
either  by  observation  with  the  fluorescent  screen  or  with  the 
photographic  plate. 

From  what  has  now  been  said  it  is  obvious  that  the  X-rays 
cannot  be  concentrated  by  lenses ;  neither  a  large  lens  of  hard 
rubber  nor  a  glass  lens  having  any  influence  upon  them.  The 
shadow-picture  of  a  round  rod  is  darker  in  the  middle  than  at 
the  edge ;  while  the  image  of  a  tube  which  is  filled  with  a  sub- 
stance more  transparent  than  its  own  material  is  lighter  at  the 
middle  than  at  the  edge. 

8.  The  question  as  to  the  reflection  of  the  X-rays  may  be  re- 
garded as  settled,  by  the  experiments  mentioned  in  the  pre- 
ceding paragraph,  in  favor  of  the  view  that  no  noticeable  regu- 
lar reflection  of  the  rays  takes  place  from  any  of  tlie  substances 
examined.  Other  experiments,  which  I  here  omit,  lead  to  the 
same  conclusion. 

One  observation  in  this  connection  should,  liowever,  be  men- 
tioned, as  at  first  sight  it  seems  to  prove  the  opposite.  I  ex- 
posed to  the  X-rays  a  photographic  plate  which  was  protected 
from  the  light  by  black  paper,  and  the  glass  side  of  which  was 
turned  towards  the  discharge  -  tube  giving  the  X-rays.  The 
sensitive  film  was  covered,  for  the  most   part,  with  polished 

8 


RONTGEN    RA.YS 

plates  of  platinum,  lead,  zinc,  and  aluminium  arranged  in  the 
form  of  a  star.  On  the  developed  negative  it  was  seen  plainly 
that  the  darkening  under  the  platinum,  the  lead,  and  particu- 
larly the  zinc,  was  stronger  than  under  the  other  plates,  the 
aluminium  having  exerted  no  action  at  all.  It  appears,  there- 
fore, that  these  three  metals  reflect  the  rays.  Since,  however, 
other  explanations  of  the  stronger  darkening  are  conceivable, 
in  a  second  experiment,  in  order  to  be  sure,  I  placed  between 
the  sensitive  film  and  the  metal  plates  a  piece  of  thin  alumin- 
ium-foil, which  is  opaque  to  ultra-violet  rays,  but  is  very  trans- 
parent to  the  X-rays.  Since  the  same  result  substantially  was 
again  obtained,  the  reflection  of  X-rays  from  the  metals  above 
named  is  proved. 

If  we  compare  this  fact  with  the  observation  already  men- 
tioned that  powders  are  as  transparent  as  coherent  masses,  and 
with  the  further  fact  that  bodies  with  rough  surfaces  behave 
like  polished  bodies  with  reference  to  the  passage  of  the  X-rays, 
as  shown  also  in  the  last  experiment,  we  are  led  to  the  con- 
clusion already  stated  that  regular  reflection  does  not  take 
place,  but  that  bodies  behave  towards  the  X-rays  as  turbid 
media  do  towards  light. 

Since,  moreover,  I  could  detect  no  evidence  of  refraction  of 
these  rays  in  passing  from  one  medium  into  another,  it  would 
seem  that  X-rays  move  with  the  same  velocity  in  all  substances ; 
and,  further,  that  this  speed  is  the  same  in  the  medium  which 
is  present  everywhere  in  sj)ace  and  in  which  the  particles  of 
matter  are  imbedded.  These  particles  hinder  the  propagation 
of  the  X-rays,  the  effect  being  greater,  in  general,  the  more 
dense  the  substance  concerned. 

9.  Accordingly  it  might  be  possible  that  the  arrangement  of 
particles  in  the  substance  exercised  an  influence  on  its  trans- 
parency; that,  for  instance,  a  piece  of  calcite  might  be  trans- 
parent in  different  degrees  for  the  same  thickness,  according  as 
it  is  traversed  in  the  direction  of  the  axis,  or  at  right  angles  to 
it.  Experiments,  however,  on  calcite  and  quartz  gave  a  nega- 
tive result. 

10.  It  is  well  known  tjuit  Lenard  came  to  the  conclusion, 

9 


xMEMOIRS    ON 

from  the  results  of  his  beautiful  experiments  on  the  transmis- 
sion of  the  cathode  rays  of  Hittorf  through  a  thin  sheet  of 
aluminium,  that  these  rays  are  phenomena  of  the  ether,  and 
that  they  diffuse  themselves  through  all  bodies.  We  can  say 
the  same  of  our  rays. 

In  his  most  recent  research,  Lenard  has  determined  the  ab- 
sorptive power  of  different  substances  for  the  cathode  rays,  and, 
among  others,  has  measured  it  for  air  from  atmospheric  press- 
ure to  4.10,  3.40,  3.10,  referred  to  1  centimetre,  according  to 
the  rarefaction  of  the  gas  contained  in  the  discharge-apparatus. 
Judging  from  the  discharge  -  pressure  as  estimated  from  the 
sparking  distance,  I  have  had  to  do  in  my  experiments  for  the 
most  part  with  rarefactions  of  the  same  order  of  magnitude, 
and  only  rarely  with  less  or  greater  ones.  I  have  succeeded  in 
comparing  by  means  of  the  L.  Weber  photometer — I  do  not 
possess  a  better  one^ — the  intensities,  taken  in  atmospheric  air, 
of  the  fluorescence  of  my  screen  at  two  distances  from  the  dis- 
charge-apparatus— about  100  and  200  millimetres;  and  I  have 
found  from  three  experiments,  which  agree  very  well  with  each 
other,  that  the  intensities  vary  inversely  as  the  squares  of  the 
distances  of  the  screen  from  the  discharge-apparatus.  Accord- 
ugly,  air  absorbs  a  far  smaller  fraction  of  the  X-rays  than  of 
le  cathode  rays.  This  result  is  in  entire  agreement  with  the 
c  ervation  mentioned  above,  that  it  is  still  possible  to  detect 
the  fluorescent  light  at  a  distance  of  2  metres  from  the  dis- 
charge-apparatus. 

Other  substances  behave  in  general  like  air;  they  are  more 
transparent  to  X-rays  than  to  cathode  rays. 

11.  A  further  difference,  and  a  most  important  one,  between 
the  behavior  of  cathode  rays  and  of  X-rays  lies  in  the  fact 
that  I  have  not  succeeded,  in  spite  of  many  attempts,  in  ob- 
taining a  deflection  of  the  X-rays  by  a  magnet,  even  in  very  in- 
tense fields. 

The  possibility  of  deflection  by  a  magnet  has,  up  to  the  pres- 
ent time,  served  as  a  characteristic  property  of  the  cathode 
rays;  although  it  was  observed  by  Hertz  and  Lenard  that  tliere 
are  different  sorts  of  cathode  rays,  ''  wliicli  are  distinguished 

10 


RONTGEN    RAYS 

from  each  other  by  their  production  of  phosphoresceuce,  by  the 
amount  of  their  absorption,  and  by  the  extent  of  their  deflec- 
tion by  a  magnet/^  A  considerable  deflection,  however,  was 
noted  in  all  of  the  cases  investigated  by  them  ;  so  that  I  do  not 
think  that  this  characteristic  will  be  given  up  except  for  strin- 
gent reasons. 

12.  According  to  experiments  especially  designed  to  test  the 
question,  it  is  certain  that  the  spot  on  the  wall  of  the  dis- 
charge-tube which  fluoresces  the  strongest  is  to  be  considered 
as  the  main  centre  from  which  the  X-rays  radiate  in  all  direc- 
tions. The  X-rays  proceed  from  that  spot  where,  according 
to  the  data  obtained  by  different  investigators,  the  cathode 
rays  strike  the  glass  wall.  If  the  cathode  rays  within  the  dis- 
charge-apparatus are  deflected  by  means  of  a  magnet,  it  is  ob- 
served that  the  X-rays  proceed  from  another  spot — namely, 
from  that  which  is  the  new  terminus  of  the  cathode  rays. 

For  this  reason,  therefore,  the  X-rays,  which  it  is  impossible 
to  deflect,  cannot  be  cathode  rays  simply  transmitted  or  re- 
flected without  change  by  the  glass  wall.  The  greater  density 
of  the  gas  outside  of  the  discharge-tube  certainly  cannot  ac- 
count for  the  great  difference  in  the  deflection,  according  to 
Lenard.  . 

I  therefore  reach  the  conclusion  that  the  X-rays  are  not 
identical  with  the  cathode  rays,  but  that  they  are  produced  by 
the  cathode  rays  at  the  glass  wall  of  the  discharge-apj^aratus. 

13.  This  production  does  not  take  place  in  glass  alone,  but, 
as  I  have  been  able  to  observe  in  an  apparatus  closed  by  a 
plate  of  aluminium  2  millimetres  thick,  in  this  metal  also. 
Other  substances  are  to  be  examined  later. 

14.  The  justification  for  calling  by  the  name  ^^  rays  "the 
agent  which  proceeds  from  the  wall  of  the  discharge-apparatus 
I  derive  in  part  from  the  entirely  regular  formation  of  shad- 
ows, which  are  seen  when  more  or  less  transparent  bodies  are 
brought  between  the  apparatus  and  the  fluorescent  screen  (or 
the  photographic  plate). 

I  have  observed,  and  in  part  photographed,  many  shadow- 
pictures  of  this  kind,  the  production  of  which  has  a  particular 

11 


MEMOIRS    ON 

charm.  I  possess,  for  instance,  photographs  of  the  shadow  of 
the  profile  of  a  door  which  separates  the  rooms  in  which,  on 
one  side,  the  discharge-apparatus  was  placed,  on  the  other  the 
photographic  plate ;  the  shadow  of  the  bones  of  the  liand ;  the 
shadow  of  a  covered  wire  wrapped  on  a  wooden  sjoool ;  of  a  set 
of  weights  enclosed  in  a  box ;  of  a  galvanometer  in  which  the 
magnetic  needle  is  entirely  enclosed  by  metal;  of  a  piece  of 
metal  whose  lack  of  homogeneity  becomes  noticeable  by  means 
of  the  X-rays,  etc. 

Another  conclusive  proof  of  the  rectilinear  propagation  of 
the  X-rays  is  a  pin-hole  photograph  which  I  was  able  to  make 
of  the  discharge-apparatus  while  it  was  enveloped  in  bhick  pa- 
per; the  picture. is  weak  but  unmistakably  correct. 

15.  I  have  tried  in  many  ways  to  detect  interference  phe- 
nomena of  the  X-rays ;  but,  unfortunately,  without  success, 
perhaps  only  because  of  their  feeble  intensity. 

16.  Experiments  have  been  begun,  but  are  not  yet  finished, 
to  ascertain  whether  electrostatic  forces  affect  the  X-rays  in 
any  way. 

17.  In  considering  the  question  what  are  the  X-rays — which, 
as  we  have  seen,  cannot  be  cathode  rays — we  may  perhaps  at 
first  be  led  to  think  of  them  as  ultra-violet  light,  owing  to  their 
active  fluorescence  and  their  chemical  actions.  But  in  so  doing 
we  find  ourselves  opposed  by  the  most  weighty  considerations. 
If  the  X-rays  are  ultra-violet  light,  this  light  must  have  the 
following  properties: 

(a)  On  passing  from  air  into  water,  carbon  disulphide,  alu- 
minium, rock-salt,  glass,  zinc,  etc.,  it  suffers  no  noticeable  re- 
fraction. 

(b)  By  none  of  the  bodies  named  can  it  be  regularly  reflected 
to  any  appreciable  extent. 

(c)  It  cannot  be  polarized  by  any  of  the  ordinary  methods. 

(d)  Its  absorption  is  influenced  by  no  other  property  of 
substances  so  much  as  by  their  density. 

That  is  to  say,  we  must  assume  that  these  ultra-violet  rays 
behave  entirely  differently  from  the  ultra -red,  visible,  and 
ultra-violet  rays  which  have  been  known  up  to  this  time. 

13 


ROXTGEX    RAYS 

I  have  been  nnable  to  come  to  this  conclusion,  and  so  have 
songht  for  another  explanation. 

There  seems  to  exist  some  kind  of  relationship  between  the 
new  rays  and  light  rays ;  at  least  this  is  indicated  by  the  for- 
mation of  shadows,  the  fluorescence  and  the  chemical  action 
produced  by  them  both.  Xow,  we  have  known  for  a  long  time 
that  there  can  be  in  the  ether  longitudinal  vibrations  besides 
the  transverse  light-vibrations ;  and,  according  to  the  views  of 
different  physicists,  these  vibrations  must  exist.  Their  exist- 
ence, it  is  true,  has  not  been  proved  up  to  the  present,  and 
consequently  their  properties  have  not  been  investigated  by 
experiment. 

Ought  not,  therefore,  the  new  rays  to  be  ascribed  to  longitu- 
dinal vibrations  in  the  ether  ? 

I  must  confess  that  in  the  course  of  the  investigation  I  have 
become  more  and  more  confident  of  the  correctness  of  this  idea, 
and  so,  therefore,  permit  myself  to  announce  this  conjecture, 
although  I  am  perfectly  aware  that  the  explanation  given  still 
needs  further  confirmation. 

WuRZBURG,  Physikalisches  Institut  der  Universitat. 
December^  1895. 


SECOXD    COMMUXICA  TIOX 

Since  my  work  must  be  interrupted  for  several  weeks,  I  take 
the  opportunity  of  presenting  in  the  following  paper  some  new 
phenomena  which  I  have  observed. 

18.  It  was  known  to  me  at  the  time  of  my  first  publication 
that  X-rays  can  discharge  electrified  bodies  ;  and  I  conjecture 
that  in  Lenard's  experiments  it  was  the  X-rays,  and  not  the 
cathode  rays,  which  had  passed  unchanged  through  the  alu- 
minium window  of  his  apparatus,  which  produced  the  action 
described  by  him  upon  electrified  bodies  at  a  distance.  I  have, 
however,  delayed  the  publication  of  my  experiments  until  I 
could  contribute  results  which  are  free  from  criticism. 

These  results  can  be  obtained  only  when  the  observations  are 
made  in  a  space  which  is  protected  completely,  not  only  from 

13 


MEMOIRSON 

the  electrostatic  forces  proceeding  from  the  vacuum-tnbe,  from 
the  conducting  wires,  from  the  induction  apparatus,  etc.,  but  is 
also  closed  against  air  which  comes  from  the  neighborhood  of 
the  discharge-apparatus. 

To  secure  these  conditions  I  had  a  chamber  made  of  zinc 
plates  soldered  together,  which  was  large  enough  to  contain 
myself  and  the  necessary  apparatus,  which  could  be  closed  air- 
tight, and  which  was  provided  with  an  opening  which  could 
be  closed  by  a  zinc  door.  The  wall  opposite  the  door  was  for 
the  most  part  covered  with  lead.  At  a  place  near  the  dis- 
charge-apparatus, which  was  set  up  outside  the  case,  the  zinc 
wall,  together  with  the  lining  of  sheet-lead,  was  cut  out  for  a 
width  of  4  centimetres;  and  the  opening  was  covered  again 
air-tight  with  a  thin  sheet  of  aluminium.  The  X-rays  pene- 
trated through  this  window  into  the  observation  space. 

I  observed  the  following  phenomena  : 

(a)  Electrified  bodies  in  air,  charged  either  positively  or 
negatively,  are  discharged  if  X-rays  fall  upon  them ;  and  this 
process  goes  on  the  more  rapidly  the  more  intense  the  rays  are. 
The  intensity  of  the  rays  was  estimated  by  their  action  on  a 
fluorescent  screen  or  a  photographic  plate. 

It  is  immaterial  in  general  whether  the  electrified  bodies  are 
conductors  or  insulators.  Up  to  the  present  I  have  not  found 
any  specific  difference  in  the  behavior  of  different  bodies  with 
reference  to  the  rate  of  discharge ;  nor  as  to  the  behavior  of 
positive  and  negative  electricity.  Yet  it  is  not  impossible  that 
small  differences  may  exist. 

{!))  If  the  electrified  conductor  be  surrounded  not  by  air  but 
l)y  a  solid  insulator,  c.  g.  paraffin,  the  radiation  has  the  same 
action  as  would  result  from  exposure  of  the  insulating  envelope 
to  a  flame  connected  to  the  earth. 

{c)  If  this  insulating  envelope  be  surrounded  by  a  close- 
fitting  conductor  which  is  connected  to  tlie  earth,  and  which, 
like  tlic  insulator,  is  transparent  to  X-rays,  the  radiation  pro- 
duces on  the  inner  electrified  conductor  no  action  which  can 
be  detected  by  my  apparatus. 

{(1)  The  observations  noted  under  (a),  (/>),  (r)  indicate  that 

14 


RONTGEN    RAYS 

air  through  which  X-rays  have  passed  possesses  the  power  of 
discharging  electrified  bodies  with  which  it  conies  in  con- 
tact. 

(e)  If  this  is  really  the  case,  and  if,  further,  the  air  retains 
this  property  for  some  time  after  it  has  been  exposed  to  the 
X-rays,  then  it  must  be  possible  to  discharge  electrified  bodies 
which  have  not  been  themselves  exposed  to  the  rays,  by  con- 
ducting to  them  air  which  has  thus  been  exposed. 

We  may  convince  ourselves  in  various  ways  that  this  con- 
clusion is  correct.  One  method  of  experiment,  although  per- 
haps not  the  simplest,  I  shall  describe. 

I  used  a  brass  tube  3  centimetres  wide  and  45  centimetres 
long;  at  a  distance  of  some  centimetres  from  one  end  a  part 
of  the  wall  of  the  tube  was  cut  away  and  replaced  by  a  thin 
aluminium  plate ;  at  the  other  end,  through  an  air-tight  cap,  a 
brass  ball  fastened  to  a  metal  rod  was  introduced  into  the  tube 
in  such  a  manner  as  to  be  insulated.  Between  the  ball  and  the 
closed  end  of  the  tube  there  was  soldered  a  side-tube  which 
could  be  connected  with  an  exhaust-apparatus;  so  that  when 
this  is  in  action  the  brass  ball  is  subjected  to  a  stream  of  air 
which  on  its  way  through  the  tube  has  passed  by  the  alumin- 
ium window.  The  distance  from  the  window  to  the  ball  was 
over  20  centimetres. 

I  arranged  this  tube  inside  the  zinc  chamber  in  such  a  posi- 
tion that  the  X-rays  could  enter  through  the  aluminium  win- 
dow of  the  tube  perpendicular  to  its  axis.  The  insulated  ball 
lay  then  in  the  shadow,  out  of  the  range  of  the  action  of  these 
rays.  The  tube  and  the  zinc  case  were  connected  by  a  con- 
ductor, the  ball  was  joined  to  a  Hankel  electroscope. 

It  was  now  observed  that  a  charge  (either  positive  or  nega- 
tive) given  to  the  ball  was  not  influenced  by  the  X-rays  so 
long  as  the  air  remained  at  rest  in  the  tube,  but  that  the 
charge  instantly  decreased  considerably  if  by  exhaustion  the 
air  which  had  been  subjected  to  the  rays  was  drawn  past  the 
ball.  If  by  means  of  storage  cells  the  ball  was  maintained  at  a 
constant  potential,  and  if  the  modified  air  was  drawn  continu- 
ously through  the  tube,  an  electric  current   arose  just  as  if 

15 


MEMOIRS    ON 

the  ball  were  connected  to  the  wall  of  the  tube  by  a  poor 
conductor. 

( /)  The  question  arises.  How  does  the  air  lose  the  property 
which  is  given  it  by  the  X-rays  ?  It  is  not  yet  settled  whether 
it  loses  this  property  gradually  of  itself — i.  e.,  without  coming 
in  contact  with  other  bodies.  On  the  other  hand,  it  is  certain 
that  a  brief  contact  with  a  body  of  large  surface,  which  does 
not  need  to  be  electrified,  can  make  the  air  inactive.  For  in- 
stance, if  a  thick  enough  stopper  of  wadding  is  pushed  into  the 
tube  so  far  that  the  modified  air  must  pass  through  it  before  it 
reaches  the  electrified  ball,  the  charge  on  the  ball  remains  un- 
affected even  while  the  exhaustion  is  taking  jolace. 

If  the  wad  is  in  front  of  the  aluminium  window,  the  result 
obtained  is  the  aame  as  it  would  be  without  the  wad;  a  proof 
that  it  is  not  particles  of  dust  which  are  the  cause  of  the  ob- 
served discharge. 

Wire  gratings  act  like  wadding;  but  the  gratings  must  be 
very  fine,  and  many  layers  must  be  placed  over  each  other  if 
the  modified  air  is  to  be  inactive  after  it  is  drawn  through 
them.  If  these  gratings  are  not  connected  to  the  earth,  as  has 
been  assumed,  but  are  connected  to  a  source  of  electric- 
ity at  a  constant  potential,  I  have  always  observed  exactly 
what  I  had  expected ;  but  these  experiments  are  not  yet  com- 
pleted. 

{g)  If  the  electrified  bodies,  instead  of  beiug  in  air,  are 
placed  in  dry  hydrogen,  they  are  also  discharged  by  the  X-rays. 
The  discharge  in  hydrogen  seemed  to  me  to  proceed  somewhat 
more  slowly;  yet  this  is  still  uncertain  on  account  of  the  diffi- 
culty of  obtaining  exactly  equal  intensities  of  the  X  -  rays  in 
consecutive  experiments. 

The  method  of  filling  the  apparatus  with  hydrogen  precludes 
the  possibility  that  the  layer  of  air  which  was  originally  pres- 
ent, condensed  on  the  surface  of  the  bodies,  played  any  im- 
portant r6le. 

{h)  In  spaces  which  are  highly  exhausted  the  discharge  of  a 
body  by  the  direct  incidence  of  X-rays  proceeds  much  more 
slowly — in  one  case  about  seventy  times  more  slowly — than  in 

16 


ROXTGEX    RAYS 

the  same  vessels  when  filled  with  air  or  hydrogen  at  atmos- 
pheric pressure. 

(i)  Experiments  are  about  to  be  begun  on  the  behavior  of  a 
mixture  of  chlorine  and  hydrogen  under  the  influence  of  X- 
rays. 

( /)  In  conclusion  I  would  like  to  mention  that  the  results 
of  investigations  on  the  discharging  action  of  X-rays  in  which 
the  influence  of  the  surrounding  gas  is  not  taken  into  account 
should  be  received  with  great  caution. 

19.  It  is  advantageous  in  many  cases  to  include  a  Tesla  ap- 
paratus (condenser  and  transformer)  between  the  discharge- 
apparatus  which  furnishes  the  X-rays  and  the  induction-coil. 
This  arrangement  has  the  following  advantages:  first,  the  dis- 
charge-apparatus is  less  easily  penetrated  and  is  less  heated; 
second,  the  vacuum  maintains  itself  for  a  longer  time,  at  least 
in  my  self-constructed  apparatus;  third,  many  discharge-tubes 
under  these  conditions  give  more  intense  X-rays.  With  tubes 
which  have  not  been  exhausted  sufficiently  or  have  been  ex- 
hausted too  much  to  be  driven  satisfactorily  by  the  induction- 
coil  alone,  the  addition  of  the  Tesla  transformer  renders  good 
service. 

The  question  immediately  arises — and  I  allow  myself  to  men- 
tion it  without  being  able  to  contribute  anything  to  its  solu- 
tion at  present — whether  X-rays  can  be  produced  by  a  con- 
tinuous discharge  under  constant  difference  of  potential;  or 
whether  variations  of  this  potential  are  essential  and  neces- 
sary for  the  production  of  the  rays. 

20.  In  paragraph  13  of  my  first  memoir  I  announced  that 
X-rays  could  originate  not  only  in  glass,  but  in  aluminium  also. 
In  the  continuation  of  my  ex^^eriments  in  this  direction  I  have 
not  found  any  solid  body  which  cannot,  nnder  the  action  of  the 
cathode  rays,  produce  X-rays.  There  is  also  no  reason  known 
to  me  why  liquids  and  gases  may  not  behave  in  the  same  man- 
ner. 

Quantitative  differences  in  the  behavior  of  different  substances 
have  appeared,  however.      If,  for  instance,  the  cathode  rays 
fall  upon  a  plate  one  half  of  which  is  made  of  platinum  0.3 
B  17 


MEMOIRS  ON  RONTGEN  RAYS 

millimetre  thick,  the  other  half  of  aluminium  1  millimetre 
thick,  we  see  on  the  photographic  image  of  this  double  plate, 
taken  by  means  of  a  pin-hole  camera,  that  the  platinum  sends 
out  many  more  X-rays  from  the  side  struck  by  the  cathode 
rays  (the  front  side)  than  does  the  aluminium  from  the  same 
side.  However,  from  the  rear  side  the  platinum  emits  prac- 
tically no  X-rays,  while  the  aluminium  sends  out  relatively 
many.  These  last  rays  are  produced  in  the  front  layers  of  the 
aluminium  and  pass  through  the  plate. 

We  can  easily  devise  an  explanation  of  this  observation,  yet 
it  may  be  advisable  to  learn  other  properties  of  the  X-rays  be- 
fore so  doing. 

It  must  be  mentioned,  however,  that  there  is  a  practical 
importance  in  the  facts  observed.  For  the  production  of  the 
most  intense  X-rays  platinum  is  best  suited,  according  to  my 
exj)eriments  up  to  the  present.  I  have  used  for  some  weeks 
with  great  success  a  discharge-apparatus  in  which  the  cathode 
is  a  concave  mirror  of  aluminium,  and  the  anode  is  a  plate  of 
platinum  placed  at  the  centre  of  curvature  of  the  mirror  and 
inclined  to  the  axis  of  the  mirror  at  an  angle  of  45°. 

21.  The  X-rays  proceed  in  this  case  from  the  anode.  I  must 
conclude,  though,  from  experiments  with  apparatus  of  different 
kinds  that  it  is  entirely  immaterial,  so  far  as  the  intensity  of 
the  X-rays  is  concerned,  whether  the  place  where  the  rays  are 
produced  is  the  anode  or  not. 

A  discharge  -  apparatus  was  prepared  specially  for  experi- 
ments with  the  alternating  currents  of  the  Tesla  transformer ;  in 
it  both  electrodes  were  aluminium  concave  mirrors  whose  axes 
were  at  right  angles;  at  their  common  centre  of  curvature 
there  was  placed  a  platinum  plate  to  receive  the  cathode  rays. 
Further  information  will  be  given  later  as  to  the  usefulness  of 
this  apparatus. 

WiJRZBDRG,  Physikalisches  lustitut  der  Universitat. 
March  9,  1896. 

18 


FURTHER  OBSERVATIONS  ON  THE  PROP- 
ERTIES OF  THE   X-RAYS 

BY 

W.  C.  ROXTGEX 


THIRD    COMMUNICATION 

Sitzungshericht  der  Koniglichen  joreussischen  Akademie  der  Wissenscliaften  zu 

BerUii,  1897 — Wiedemann,  Annalen  der  Physik  und 

der  Chemie,  64,  1898. 


CONTEXTS 

PAGE 

Diffusion  of  X-Rays 21 

Conditions  Influencing  Fluorescence 24 

Intensity  in  Biff'erent  Directions 24 

Transparency ;  Selective  Absorption 26 

Absorption  with  Different  Tubes;  with  Different  Interrupters,  etc 30 

''Hard''  and  "Soft "  Tubes 32 

Connection  between  X-Bays  and  Cathode  Bays 35 

Absorption  by  Crystals 39 

Optical  Effect  of  X-Bays 39 

Diffraction  of  X-Bays 40 


FURTHER  OBSERVATIONS  ON  THE  PROP- 
ERTIES  OF  THE  X-RAYS 


BY 

W.  C.  ROXTGEX 


THIR  D    CO  MM  UNI  CATIO  N 

1.  If  an  opaqne  plate  be  placed  betvreen  a  discharge-appara- 
tus* which  is  emitting  intense  X-rajs  and  a  fluorescent  screen, 
in  such  a  position  that  it  shades  the  entire  screen,  there  may  still 
be  noticed,  in  spite  of  the  plate,  an  illumination  of  the  barium 
platino-cyanide.  This  illumination  can  be  seen  even  when  the 
screen  lies  directly  on  the  plate;  and  one  is  inclined  at  first 
sight  to  consider  the  plate  as  transparent.  If,  however,  the 
screen  lying  on  the  plate  be  covered  by  a  thick  pane  of  glass, 
the  fluorescent  light  becomes  much  weaker;  and  it  vanishes 
entirely  if,  instead  of  using  a  glass  plate,  the  screen  is  sur- 
rounded by  a  cylinder  of  sheet-lead  0.1  centimetre  thick,  which 
is  closed  at  one  end  by  the  non-transparent  plate,  and  at  the 
other  by  the  head  of  the  observer. 

The  phenomenon  now  described  may  be  due  either  to  diffrac- 
tion of  rays  of  very  great  wave-length,  or  to  the  fact  that  the 
bodies  which  surround  the  discharge-apparatus  and  tlirough 
whicli  the  rays  pass,  especially  the  air,  themselves  emit  X-rays. 

*  All  the  discharge-tubes  mentioned  in  the  following  communicalion  are 
constructed  according  to  the  principle  given  in  paragraph  20  of  my  sec- 
ond paper.  The  greater  portion  of  them  I  obtained  from  the  firm  of 
Greiner  &  Friedrichs,  in  Stutzerbach  i.  Th.,  whom  I  wish  to  thank  publicly 
for  the  material  whicli  has  been  furnished  me  in  such  abundance  and 
without  expense. 

21 


MEMOIRS    ON 


The  latter  explanation  is  the  correct  one,  as  may  be  proved 
with  the  following  apparatus,  among  others  :  The  fig'ire  rep- 
resents a  very  thick-walled  glass  bell-jar,  20  centimetres  high 
and  10  centimetres  broad,  which  is  closed  by  a  thick  zinc  plate 
cemented  on.  At  1  and  2  are  inserted 
plates  of  lead  in  the  shape  of  circular 
segments ;  these  are  somewhat  larger 
than  half  the  cross -section  of  the  jar. 
and  prevent  the  X-rays,  which  enter 
through  an  opening  in  the  zinc  plate 
covered  with  a  celluloid  film,  from  reach- 
ing directly  the  space  above  the  lead 
plate,  2.  On  the  upper  side  of  this 
sheet  of  lead  there  is  fastened  a  small 
barium  platino  -  cyanide  screen,  which 
nearly  fills  the  entire  cross-section  of  the 
jar.  This  cannot  be  struck  either  by  the 
-*  direct  rays  or  by  such  as  have  suffered  a 
single  diffuse  reflection  at  a  solid  body  {e.g.,  the  glass  wall). 
The  jar  is  filled  with  dust-free  air  before  each  experiment.  If 
X-rays  are  made  to  enter  the  jar  in  such  a  manner  that  they 
are  all  received  upon  the  lead  screen  1,  no  fiuorescence  is 
observed  at  2  ;  the  fluorescent  screen  first  begins  to  light  up 
on  the  half  not  covered  by  the  lead  plate  2  only  when  by  tip- 
ping the  bell-jar  direct  radiation  reaches  the  space  between  1 
and  2.  If  the  bell-jar  is  now  connected  to  an  aspirator-pump 
worked  by  a  stream  of  water,  it  is  observed  that  the  fluores- 
cence becomes  more  and  more  weak  as  the  exhaustion  pro- 
ceeds ;  but  when  the  air  is  readmitted  the  intensity  again  in- 
creases. 

Since  now,  as  I  have  found,  the  mere  contact  with  air  whicli 
has  been  exposed  shortly  before  to  X-rays  does  not  produce  any 
sensible  fluorescence  of  the  barium  platino-cyanide,  we  must 
conclude  from  the  experiment  described  that  air  during  its  ex- 
posure to  radiation  emits  X-rays  in  all  directions. 

If  our  eyes  were  as  sensitive  to  X-rays  as  they  are  to  light- 
rays,  a  discharge-apparatus  in  operation  would  appear   to  us 


ROXTGEX    RAYS 

like  a  light  bnrning  in  a  room  moderately  filled  witli  tobacco 
smoke  ;  perhaps  the  colors  of  the  direct  rays  and  of  those 
coming  from  the  particles  of  air  might  be  different. 

The  question  as  to  whether  the  rays  emitted  by  a  body  which 
is  receiving  radiation  are  of  the  same  kind  as  those  which  are 
incident,  or,  in  other  words,  whether  the  cause  of  tliese  rays  is 
diffuse  reflection  or  a  process  like  fluorescence,  I  have  not  yet 
been  able  to  decide.  The  fact  that  the  rays  coming  from  the 
air  are  photographically  active  can  be  proved  easily  ;  and  this 
action  makes  itself  noticeable  sometimes  in  a  way  not  desired 
by  the  observer.  In  order  to  guard  against  this  action,  as  is 
often  necessary  in  long  exposures,  the  photographic  plates  must 
be  protected  by  suitable  lead  casings. 

2.  In  order  to  compare  the  intensity  of  the  radiation  of  two 
discharge  -  tubes,  and  for  various  other  experiments,  I  have 
used  an  arrangement  which  is  based  on  the  principle  of  the 
Bouguer  photometer,  and  which,  for  the  sake  of  simplicity,  I 
shall  call  a  photometer  also.  A  rectangular  sheet  of  lead  35 
centimetres  high,  150  centimetres  long,  and  0.15  centimetre 
thick,  supported  on  a  board  frame,  is  placed  vertically  in  the 
middle  of  a  long  table.  At  each  side  of  this  is  placed  a  dis- 
charge-tube, which  can  be  moved  along  the  table.  At  one  end 
of  the  lead  strip  a  fluorescent  screen*  is  so  placed  that  each 
half  receives  radiation  perpendicularly  from  one  tube  only. 
In  effecting  the  measurements,  adjustments  are  made  until 
there  is  equal  brightness  of  the  fluorescence  on  the  two  halves. 

Some  remarks  on  the  use  of  this  instrument  may  find  a  place 
here.  It  should  be  mentioned  first  that  the  settings  are  often 
made  more  difiicult  by  the  lack  of  constancy  of  the  source  of 
radiation,  the  tubes  responding  to  every  irregularity  in  the 
interruption  of  the  primary  current,  such  as  occur  with  the 

*  In  this  and  other  experiments  the  Edison  fluorescent  screen  has  proved 
most  useful.  This  consists  of  a  box  like  a  stereoscope  which  can  be  held 
light-tight  against  the  head  of  the  observer,  and  whose  card-board  end  is 
covered  with  barium  platino-cyanide.  Edison  uses  tutigstate  of  calcium 
in  place  of  barium  platiuo  -  cyanide  ;  but  I  prefer  the  latter  for  many 
reasons. 


MEMOIRS    ON 

Deprez  interrupter,  and  especially  with  the  Foucault  instru- 
ment. Repeated  settings  are  therefore  advisahle.  In  the  sec- 
ond place,  I  should  here  enumerate  the  conditions  which  in- 
fluence the  brightness  of  a  given  fluorescent  screen  struck  b}^ 
X-rays  in  such  rapid  succession  that  the  eye  of  the  observer 
can  no  lonsrer  detect  the  intermittence  of  the  radiation.  This 
briglitness  depends  (1)  wpon  the  intensity  of  the  radiation 
which  proceeds  from  the  platinum  plate  of  the  discharge-tube  ; 
{'i)  very  j^robably  upon  the  kind  of  rays  striking  the  screen, 
since  all  kinds  of  rays  (see  below)  are  not  necessarily  equally 
active  in  producing  fluorescence  ;  (3)  upon  the  distance  of  the 
screen  from  the  centre  of  emission  of  the  rays  ;  (4)  upon  the 
absorption  which  the  rays  experience  on  their  way  to  the  barium 
platino-cyanide  screen  ;  (5)  upon  the  number  of  discharges 
per  second  ;  (6)  upon  the  duration  of  each  single  discharge  ; 
(T)  upon  the  duration  and  the  strength  of  the  after-illumina- 
tion of  the  barium  platino-cyanide;  and  (8)  upon  the  radiation 
falling  on  the  screen  from  the  bodies  which  surround  the  dis- 
charge-tube. In  order  to  avoid  errors,  it  must  always  be  re- 
membered that  the  conditions  are  in  general  like  those  which 
would  exist  if  we  had  to  compare,  by  means  of  fluorescent 
action,  two  intermittent  sources  of  light  of  different  colors, 
which  are  surrounded  by  an  absorbing  envelo2)e  placed  in  a 
turbid — or  fluorescing — medium. 

3.  According  to  paragraph  i'i  of  my  flrst  communication, 
the  point  in  the  discharge -apparatus  which  is  struck  by  the 
cathode  rays  is  the  centre  of  emission  of  the  X-rays,  and  from 
this  these  rays  spread  out  'Mn  all  directions.''  It  becomes 
now  of  interest  to  determine  how  the  intensity  of  the  radiation 
varies  with  tlie  direction. 

For  this  investigation  the  discharge-tubes  best  suited  to  the 
purpose  are  those  in  the  shape  of  a  sphere,  with  smoothly 
polished  platinum  plates,  which  are  struck  by  the  cathode  rays 
at  an  angle  of  -45°.  Even  without  further  appliances  we  can 
recognize  from  the  uniformly  bright  fluorescence  of  the  hemi- 
spherical glass  wall  surrounding  the  platinum  plate  that  very 
iiroat   differences  of  intensity  in  different  directions   do   not 

34 


RONTGEN    RAYS 

exist ;  so  that  Lambert's  law  of  emission  does  not  hold  in 
this  case.  Xevertheless,  this  fluorescence  for  the  most  part 
might  still  be  due  to  the  cathode  rays. 

To  test  this  question  more  accurately,  several  tubes  were  ex- 
amined by  means  of  the  ^^hotometer  as  to  their  radiation  in 
different  directions.  Moreover,  besides  doing  this,  I  have  ex- 
posed with  the  same  object  photographic  films  bent  into  a  semi- 
circle (radius  25  centimetres)  about  the  platinum  plate  of  the 
discharge-tube  as  a  centre.  In  both  experiments,  however,  the 
varying  thickness  of  the  different  portions  of  the  walls  of  the 
tube  produced  a  disturbing  action,  because  the  X-rays,  pro- 
ceeding in  different  directions,  were  unequally  absorbed.  Yet 
by  interposing  thin  plates  of  glass  I  finally  succeeded  in  mak- 
ing the  thickness  of  glass  traversed  about  the  same. 

The  result  of  these  experiments  is  that  the  radiation  through 
an  imaginary  hemisphere,  described  around  the  platinum  plate 
as  a  centre,  is  nearly  uniform  almost  out  to  the  edge.  It  was 
not  until  the  emission  angle  of  the  rays  was  about  80°  that  I 
noticed  the  beginning  of  a  decrease  in  the  radiation  ;  and  even 
then  this  decrease  was  relatively  very  small  ;  so  that  the  main 
change  in  the  intensity  occurs  between  89°  and  90°. 

Xo  difference  in  the  kind  of  rays  emitted  at  different  angles 
have  I  been  able  to  detect. 

As  a  consequence  of  the  distribution  of  intensity  of  the  X- 
rays,  as  now  described,  the  images  of  the  platinum  plate  which 
are  received — either  on  a  fluorescent  screen  or  on  a  photo- 
graphic plate,  through  a  pin-hole  camera  or  witli  a  narrow  slit 
— must  be  more  intense  the  greater  the  angle  which  the  plati- 
num plate  makes  with  the  screen  or  with  the  photographic 
plate  ;  always  presupposing  that  this  angle  does  not  exceed 
80°.  By  means  of  suitable  appliances  which  allow  comparisons 
to  be  made  between  the  images  received  simultaneously  at  dif- 
ferent angles  from  the  same  discharge-tube,  I  have  been  able 
to  confirm  this  conclusion. 

A  similar  case  of  distribution  of  the  intensity  of  emitted  rays 
occurs  in  Optics  in  the  case  of  fluorescence.  If  a  few  drops  of 
fluorescein  solution  be  allowed  to  fall  into  a  rectans^ular  tank 


MEMOIRS    OX 

filled  with  water,  and  if  at  the  same  time  we  illuminate  the 
tank  with  white  or  with  violet  light,  we  observe  that  the 
brightest  fluorescence  proceeds  from  the  edges  of  the  threads 
of  the  slowly  sinking  fluorescein — i.  e.,  from  the  j^laces  where 
the  emission  angle  of  the  fluorescent  light  is  the  greatest.  As 
Stokes  has  remarked,  d  propos  of  a  similar  experiment,  this 
phenomenon  is  due  to  the  fact  that  the  rays  whicli  produce 
fluorescence  are  absorbed  by  the  fluorescein  solution  much 
more  strongly  than  is  the  fluorescent  light  itself.  Now  it  is 
worthy  of  note  that  the  cathode  rays,  which  produce  the 
X-rays,  are  absorbed  by  platinum  much  more  than  are  the 
X-rays,  and  it  is  easy  to  conjecture  from  this  that  a  relation- 
ship exists  between  the  two  phenomena — the  transformation  of 
ordinary  light  into  fluorescent  light,  and  that  of  cathode  rays 
into  X-rays.  A  conclusive  proof,  of  any  kind,  of  such  an  as- 
sumption is  not  known  at  the  present  time,  however. 

Moreover,  with  reference  to  the  technique  of  the  production 
of  shadow  pictures  by  means  of  X-rays,  the  observations  on  the 
distribution  of  intensity  of  the  rays  proceeding  outward  from 
the  platinum  plate  have  a  certain  importance.  According  to 
what  has  been  stated  above,  it  is  advisable  to  place  the  dis- 
charge-tube in  such  a  position  that  the  rays  used  in  producing 
the  image  shall  leave  the  platinum  plate  at  as  great  an  angle  as 
possible,  though  this  should  not  be  much  over  80°.  By  this 
means  the  sharpest  pictures  are  produced  ;  and,  if  the  platinum 
plate  be  perfectly  plane,  and  the  construction  of  the  tube  of 
such  a  kind  that  the  oblique  rays  pass  through  a  not  materially 
thicker  glass  wall  than  those  rays  which  are  emitted  perpen- 
dicular to  the  platinum  plate,  then  the  radiation  on  the  object 
suffers  no  loss  in  intensity. 

4.  I  have  designated  in  my  first  communication  by  ''trans- 
parency of  a  body  "  the  ratio  of  the  brightness  of  a  fluorescent 
screen  placed  perpendicular  to  the  rays,  and  close  behind  the 
body,  to  that  which  the  screen  shows  when  viewed  under  the 
same  conditions,  but  with  the  body  removed.  ''  Specific  trans- 
parency "  of  a  body  will  be  used  to  indicate  the  transparency  of 
the  body  reduced  to  a  thickness  of  unity  ;  this  is  equal  to  the 

2G 


RONTGEX    RAYS 

dth  root  of  the  transparency,  if  d  is  the  thickness  of  the  layer 
traversed,  measured  in  the  direction  of  the  rays. 

In  order  to  determine  the  transparency,  I  have  used  prin- 
cipally, since  my  first  communication,  the  photometer  described 
above.  The  body  to  be  investigated — aluminium,  tin-foil,  glass, 
etc.,  made  in  the  form  of  a  plate — was  placed  before  one  of  the 
two  equally  bright  fluorescent  halves  of  the  screen ;  and  the 
inequality  in  brightness  thus  produced  was  made  to  vanish, 
either  by  increasing  the  distance  of  the  radiating  discharge- 
apparatus  from  the  uncovered  half  of  the  screen,  or  by  bringing 
the  other  tube  nearer.  In  both  cases  the  correctly  measured 
ratio  of  the  squares  of  the  distances  of  the  platinnm  plates  of 
the  discharge-tubes  from  the  screen,  before  and  after  the  dis- 
placement of  the  apparatus,  is  the  desired  value  of  the  trans- 
parency of  the  interposed  body.  Both  methods  led  to  the 
same  result.  By  the  addition  of  a  second  plate  to  the  first,  the 
transparency  of  the  second  i)late  may  be  found  in  a  similar 
manner  for  rays  which  have  already  passed  throngh  one. 

The  method  above  described  presuj^poses  that  the  brightness 
of  a  fluorescent  screen  varies  inversely  as  the  square  of  its  dis- 
tance from  the  source  of  rays,  and  this  is  true,  in  the  first 
place,  only  if  the  air  neither  absorbs  nor  emits  X-rays,  and  if, 
secondly,  the  brightness  of  the  fluorescent  light  is  proportional 
to  the  intensity  of  emission  of  rays  of  the  same  kind.  The 
first  condition  is  certainly  not  satisfied,  and  it  is  doubtful 
whether  the  second  is  ;  I  convinced  myself  long  ago  by  ex- 
periment, as  already  described  in  paragraph  10  of  my  first  com- 
munication, that  the  deviations  from  the  law  of  proportion- 
ality are  so  small  that  they  can  be  safely  neglected  in  the  case 
before  us.  It  should  be  mentioned  with  reference  to  the  fact 
that  X-rays  also  proceed  from  the  irradiated  body,  first,  that  a 
difference  in  the  transparency  of  a  plate  of  aluminium  0.9'^5 
millimetre  thick,  and  of  31  aluminium  sheets  laid  upon  one 
another,  each  of  a  thickness  of  0.0299  millimetre  —  31  x 
0.0299  =  0.927  —  could  not  be  detected  with  the  photometer 
used  ;  and,  second,  that  the  brightness  of  the  fluorescent 
screen  was  not  sensibly  different  when  the  plate  was  close  in 

27 


MEMOIRS    ON 

front  of  the  screen  and  when  it  was  placed  at  a  greater  dis- 
tance from  it. 

For  aluminium,  the  results  of  this  experiment  on  trans- 
parency are  as  follows  : 

Transparency  for  Perpendicular  Rays 

Tube  2        Tube  3         Tube  i         Tube  2 

The  first  1  mm.  thick  Al.  plate  0.40  0.45  —  0.68 

Thesecondlmm.  '^       '^        "  0.55  0.68  —  0.73 

The  first  2  mm.    ^'       ''        ''         —  0.30  0.39  0.50 

The  second  2  mm.  "       *'        ''         —  0.39  0.54  0.63 

From  these  experiments,  and  from  similar  ones  on  glass  and 
tin-foil,  we  deduce  at  once  the  following  result  :  if  we  imagine 
a  substance  divided  into  layers  of  equal  thickness,  placed  per- 
pendicular to  parallel  rays,  each  of  these  layers  is  more  trans- 
parent for  the  transmitted  rays  than  the  one  before  it  ;  or,  in 
other  words,  the  specific  transparency  of  a  substance  increases 
with  its  thickness. 

This  result  is  completely  in  accord  with  what  may  be  ob- 
served in  the  photograph  of  a  tin-foil  scale  as  descril3ed  in  par- 
agraph 4  of  my  first  communication  ;  and  also  with  the  fact 
that  in  photographic  pictures  the  shadow  of  thin  sheets — ('.g.,oi 
the  paper  used  to  wrap  up  the  plate — is  proportionally  strongly 
marked. 

5.  Even  if  two  plates  of  different  substances  are  ecjually 
transparent,  this  equality  may  not  persist  when  the  thickness 
of  the  plates  is  changed  in  the  same  ratio,  nothing  else  being 
altered.  This  fact  may  be  proved  most  easily  by  the  help  of 
two  scales  placed  side  by  side  ;  for  instance,  one  of  platinum, 
the  other  of  aluminium.  I  used  for  this  purpose  platinum-foil 
0.0026  millimetre  thick,  and  aluminium  -  foil  0.0299  milli- 
metre thick.  I  brought  the  double  scale  before  the  fiuores- 
cent  screen,  or  before  a  photographic  plate,  and  allowed  rays 
to  fall  upon  it ;  I  found  in  one  case  that  a  single  sheet  of  plat- 
inum was  of  equal  transparency  with  a  six-fold  layer  of  alumin- 
ium ;  but  that  the  transparency  of  a  double  platinum  layer  was 

28 


RONTGEX    RAYS 

equal  not  to  that  of  a  twelve-fold  layer  of  alumiuiimi,  but  to  a 
sixteeen-fold  layer.  Using  another  discharge-tube,  I  obtained, 
1  platinum  —  8  aluminium  ;  8  platinum  =  90  aluminium.  It 
follows  from  these  experiments,  therefore,  that  the  ratio  of  the 
thickness  of  platinum  and  aluminium  of  equal  transparency  is 
smaller  in  j^roportion  as  the  layers  in  question  become  thicker. 

6.  The  ratio  of  the  thicknesses  of  two  equally  transparent 
plates  of  different  materials  depends  also  upon  the  thickness 
and  the  material  of  the  body — e.g.,  the  glass  wall  of  the  dis- 
charge-ajiparatus  —  which  the  rays  must  first  traverse  before 
they  reach  the  plates  in  question. 

In  order  to  prove  this  conclusion — which  is  not  surprising 
after  what  has  been  said  in  sections  4  and  5 — we  may  use  an 
arrangement  which  I  call  a  platinum-aluminium  window,  and 
which,  as  we  shall  see,  may  also  be  used  for  other  purposes. 
This  consists  of  a  rectangular  piece  (4.0  x  6.5  centimetres)  of 
platinum-foil  of  0.0026  millimetre  thickness,  which  is  cement- 
ed to  a  thin  paper  screen,  and  through  which  are  punched  15 
round  holes,  arranged  in  three  rows,  each  hole  having  a  diame- 
ter of  0.7  centimetre.  These  little  windows  are  covered  with 
panes  of  aluminium,  0.0299  millimetre  thick,  which  fit  exact- 
1}-,  and  are  carefully  superposed  in  such  a  way  that  at  the  first 
window  there  is  one  disk  ;  at  the  second,  two,  etc.  ;  finally,  at 
the  fifteenth,  fifteen  disks.  If  this  arrangement  be  brought  in 
front  of  the  fluorescent  screen,  it  may  be  observed  very  plainly, 
in  case  the  tubes  are  not  too  hard  (see  below),  how  many  alu- 
minium sheets  have  the  same  transparency  as  the  platinum- 
foil.     This  number  will  be  called  the  window-number. 

For  the  window-number  I  obtained  in  one  case  by  direct  ra- 
diation the  value  5.  A  plate  of  common  soda -glass,  2  milli- 
metres thick,  was  then  held  in  front ;  the  window-number  was 
10.  So  that  the  ratio  of  the  thickness  of  the  platinum  and  alu- 
minium sheets  of  equal  transparency  was  reduced  one -half 
when  I  used  rays  which  had  passed  through  a  plate  of  glass  2 
millimetres  thick  instead  of  using  those  coming  direct  from  the 
discharge-apparatus.     Q.  E.  D. 

The   following   experiment    also   deserves   mention   in   this 

29 


MEMOIRS    OX 

place:  The  platinum -aluminium  window  was  laid  upon  a 
small  package  which  contained  12  photographic  films,  and  was 
then  exposed  ;  after  development,  the  first  film  lying  under 
the  window  showed  the  window-number  10,  the  twelfth  the 
number  13  ;  and  the  others,  in  proper  order,  the  transition 
from  10  to  13. 

7.  The  experiments  communicated  in  sections  4,  5,  and  G 
refer  to  the  modifications  whicli  the  X-rays  coming  from  a  dis- 
charge-tube experience  on  passing  through  different  substances. 
It  will  now  be  proved  that  one  and  the  same  substance,  with 
the  same  thickness  traversed,  may  be  transparent  in  different 
degrees  to  rays  which  are  emitted  by  different  tubes. 

In  the  following  table  are  given,  for  this  purpose,  the  values 
of  the  transparency  of  an  aluminium  plate  2  millimetres  thick 
for  rays  produced  in  different  tubes.  Some  of  these  values  are 
taken  from  the  first  table  on  page  28  : 

Tbansparency  for  Perpendicular  Radiation 

Tubes 

1  2  3  4  2  5 

of  an  Al.  plate  2  mm.  thick,  0.0044   0.22    0.30    0.39    0.50    0.50 

The  discharge  -  tubes  are  not  materially  different  in  their 
construction  or  in  the  thickness  of  their  glass  walls,  but  vary 
mainly  in  tlie  degree  of  exhaustion  of  the  contained  gas  and  in 
the  discharge -potential  which  is  conditioned  by  this;  tube  1 
requires  the  lowest,  tube  5  the  highest,  potential ;  or,  as  we 
shall  say,  to  be  brief,  tube  1  is  the  *' softest,'"  tube  5  the 
*^ hardest."  The  same  induction-coil  —  in  direct  connection 
with  the  tubes — the  same  interrupter,  and  the  same  strength  of 
current  in  the  primary  were  used  in  all  the  cases. 

All  the  many  other  bodies  which  I  have  investigated  behave 
in  the  same  manner  as  aluminium  ;  all  are  more  transparent 
for  the  rays  of  a  harder  tube  than  for  those  of  a  softer  one.'*' 
This  fact  seems  to  me  to  be  worthy  of  special  consideration. 

*  See  below  for  the  behavior  of  "  nou-normal  "  tubes. 
30 


RONTGEN    RAYS 

The  ratio  of  the  thicknesses  of  two  equally  transparent 
plates  of  different  substances  is  also  dependent  upon  the  hard- 
ness of  the  tube  used.  This  may  be  recognized  immediately 
with  the  platinum-aluminium  window  (§  5)  ;  with  a  very  soft 
tube,  for  example,  the  window-number  may  be  found  to  be  2  ; 
while  with  a  tube  which  is  very  hard,  but  otherwise  the  same, 
the  scale  which  reaches  No.  15  does  not  extend  far  enough. 
This  means,  then,  that  the  ratio  of  the  thicknesses  of  platinum 
and  aluminium  of  equal  transparency  is  smaller  in  proportion 
as  the  tubes  from  Avhich  the  rays  come  are  harder,  or — with 
reference  to  the  result  reported  above — as  the  rays  are  less 
easily  absorbed. 

The  different  behavior  of  rays  produced  in  tubes  of  different 
hardness  is  self-evident  also  in  the  familiar  shadow-pictures  of 
hands,  etc.  With  a  very  soft  tube,  dark  pictures  are  obtained 
in  which  the  bones  are  not  very  prominent  ;  by  using  a  harder 
tube  the  bones  are  very  plain  and  all  the  details  are  visible,  the 
soft  parts,  on  the  contrary,  being  weak;  while  with  an  ex- 
tremely hard  tube  only  faint  shadows  are  obtained,  even  of  the 
bones.  From  what  has  been  said  it  follows  that  the  choice  of 
the  tube  to  be  used  must  depend  upon  the  constitution  of  the 
object  to  be  pictured. 

8.  It  still  remains  to  note  that  the  quality  of  the  rays  fur- 
nished by  one  and  the  same  tube  depends  upon  a  variety  of 
conditions.  As  the  investigation  made  with  the  platinum- 
aluminium  window  shows,  this  is  influenced  :  (1)  By  the  man- 
ner and  perfection  with  which  the  Deprez  or  Foucault  inter- 
rupter* works — i.  e.,  by  the  variation  of  the  primary  current ; 
to  this  belongs  the  phenomenon  so  often  observed,  that  single 
discharges  out  of  a  rapid  succession  produce  X-rays  which  are 
not  only  particularly  intense,  but  which  are  distinguished  from 
the  others  by  the  [^slight^  extent  to  which  they  are  absorbed  ;  (2) 
by  a  spark-gap  which  is  included  in  the  secondary  circuit  of 
the  discharge-apparatus ;  (3)  by  including  in  the  circuit  a  Tesla 


*  A  good  Deprez  interrapter  works  more  regularly  than  a  Foucault 
apparatus ;  the  latter,  however,  utilizes  the  primary  current  better. 

31 


MEMOIRS    ON 

transformer  ;  (4)  by  the  degree  of  exhaustion  of  the  discharge- 
ajjparatus  (as  already  mentioned)  ;  (5)  by  different  conditions 
in  the  interior  of  the  discharge-tube,  which  are  not  yet  suffi- 
ciently understood.  Several  of  these  factors  deserve  a  some- 
what more  extended  consideration. 

If  we  take  a  tube  which  has  not  yet  been  used,  nor  even  ex- 
hausted, and  connect  it  to  the  mercury-pump,  we  shall  obtain, 
after  the  necessary  pumping  and  heating,  such  a  degree  of  ex- 
haustion that  the  first  X-rays  are  noticeable  by  means  of  the 
feeble  illumination  of  the  fluorescent  screen  lying  near.  A 
si^ark-gap  in  parallel  with  the  tube  gives  sparks  only  a  few 
millimetres  long,  the  platinum-aluminium  window  shows  only 
very  low  numbers,  the  rays  are  easily  absorbed.  The  tube  is 
*^  very  soft."  If  now  the  spark-gap  be  put  in  series,  or  a  Tesla 
transformer  be  inserted,*  rays  are  emitted  which  are  more 
intense  and  less  easily  absorbed.  I  found,  for  example,  in  one 
case,  that  by  increasing  the  series  spark-gap  the  window-num- 
ber could  be  gradually  brought  from  2.5  to  10. 

(These  observations  suggested  the  question  whether  at  still 
higher  pressures  X-rays  could  not  be  obtained  by  the  use  of  a 
Tesla  transformer.  This  is,  in  fact,  the  case  :  using  a  narrow 
tube  with  wire-shaped  electrodes,  I  could  still  observe  X-rays 
when  the  pressure  of  the  enclosed  air  amounted  to  3.1  milli- 
metres of  mercury.  If  hydrogen  were  used  instead  of  air,  the 
pressure  could  be  even  higher.  The  lowest  pressure  at  which 
X-rays  can  be  produced  in  air  I  have  not  been  able  to  deter- 
mine; it  is  in  many  cases  less  than  0.0002  millimetre  of  mer- 
cury ;  so  that  the  limits  of  pressure  within  which  X-rays  may 
arise  are  even  now  very  considerable.) 

Further  exhaustion  of  a  ^'very  sof t  "  tube — connected  di- 
rectly to  the  induction-coil — results  in  the  radiation  becoming 
more  intense,  and  in  a  greater  fraction  of  it  passing  through 


*  The  fact  tbat  a  spark-gap  in  series  has  the  same  effect  as  a  Tesla  trans- 
former I  was  able  to  mention  in  the  French  edition  of  my  second  com- 
munication {Arch,  des  Sci.  Physique,  etc.,  de  Oen^ve,  1896);  in  the  German 
publication  this  remark  was  omitted  by  accident. 

32 


RONTGEN    RAYS 

the  bodies  on  which  it  falls  :  a  hand  held  in  front  of  the  fluo- 
rescent screen  is  more  transparent  than  before,  and  the  plati- 
nnm-aluminium  window  gives  a  higher  window-number.  At 
the  same  time  the  spark-gap  in  parallel  with  the  tube  must  be 
increased  "in  length  in  order  to  send  the  discharge  through  the 
tube  :  the  tube  has  become  "  harder/'  If  the  tube  is  exhaust- 
ed still  more,  it  becomes  so  ''  hard  "  that  the  spark-gap  must 
be  made  more  than  20  centimetres  long  ;  and  now  the  tube 
emits  rays  for  which  substances  are  unusually  transparent : 
plates  of  iron  4  centimetres  thick,  for  example,  being  seen  to 
be  transparent  when  viewed  with  the  fluorescent  screen. 

The  behavior,  as  now  given,  of  a  tube  directly  connected 
both  with  the  pump  and  the  induction-coil  is  the  normal  one  ; 
but  there  often  occur  variations  which  are  caused  by  the  dis- 
charges themselves.  The  conduct  of  the  tubes  is  in  many 
cases  quite  unaccountable. 

We  have  supposed  the  tube  to  become  hard  owing  to  con- 
tinued exhaustion  by  the  pump  ;  this  may  happen  in  another 
way.  A  fairly  hard  tube,  sealed  off  from  the  pump,  becomes 
of  itself  continually  harder — unfortunately  for  the  duration  of 
its  usefulness — even  when  it  is  used  in  the  proper  way  for  the 
production  of  X-rays  ;  that  is  to  say,  when  discharges  are  sent 
through  it  which  do  not  cause  the  platinum  to  glow,  or  at  least 
only  faintly.     A  gradual  self-exhaustion  takes  place. 

With  such  a  tube,  which  has  become  hard  in  this  way,  I  have 
obtained  a  most  beautiful  photographic  shadow-picture  of  the 
double  barrels  of  a  hunting-rifle  w^th  cartridges  in  place,  in 
which  all  the  details  of  the  cartridges,  the  internal  faults  of  the 
damask  barrels,  etc.,  could  be  seen  most  distinctly  and  sharply. 
The  distance  from  the  platinum  plate  of  the  discharge -tube 
to  the  photographic  plate  was  15  centimetres,  the  time  of 
exposure  was  12  minutes  —  comparatively  long  owing  to  the 
small  photographic  action  of  these  rays,  which  are  less  absorb- 
able (see  below).  The  Deprez  interrupter  must  be  rej^laced  by 
the  Foucault  apparatus.  It  would  be  of  interest  to  construct 
tubes  which  require  still  higher  potentials  to  be  used  than  has 
been  possible  up  to  the  present  time, 
c  33 


MEMOIRS    ON 

As  to  the  cause  of  ii  tube's  becoming  hard  when  sealed  off 
from,  the  pump,  the  explanation  given  above  is  the  self  -  ex- 
haustion of  the  tube  owing  to  the  discharges.  But  this  is  not 
the  only  cause  ;  there  are  also  changes  at  the  electrodes  which 
influence  the  result.     What  they  consist  in  1  do  not  know. 

A  tube  which  has  become  too  hard  can  be  made  softer  by  ad- 
mission of  air,  often  also  by  heating  the  tube  or  by  reversing 
the  direction  of  the  current ;  or,  finally,  by  sending  powerful 
discharges  through  it.  In  the  last  case,  however,  the  tube  has 
acquired,  for  the  most  part,  other  properties  than  those  men- 
tioned above  ;  thus  it  often  requires,  for  instance,  a  very  great 
discharge-potential,  and  yet  furnishes  rays  which  have  a  com- 
paratively small  window-number  and  which  are  easily  absorbed. 
I  need  not  continue  further  the  discussion  of  the  behavior  of 
the  "non-normal"  tubes.  The  tubes  constructed  by  Herr 
Zehnder,  having  a  vacuum  which  can  be  regulated,  since  they 
contain  a  small  piece  of  charcoal,  have  done  me  very  good 
service. 

The  observations  communicated  in  this  section,  and  others 
also,  have  led  me  to  the  opinion  that  the  composition  of  the 
rays  emitted  from  a  discharge-tube  provided  with  a  platinum 
anode  is  conditioned  essentially  upon  the  duration  of  the  dis- 
charge-current. The  degree  of  exhaustion,  the  hardness,  play 
a  part  only  because  of  this,  since  the  form  of  the  discharge  de- 
pends upon  it.  If  we  can  produce  in  any  way  whatever  the 
form  of  discharge  necessary  for  the  appearance  of  the  X-rays, 
X-rays  can  be  produced,  and  this  even  at  relatively  high 
pressures. 

In  conclusion,  it  is  worth  mentioning  that  the  quality  of  the 
rays  produced  by  a  tube  is  not  changed,  or,  at  most,  only  very 
slightly,  by  very  considerable  changes  in  the  strength  of  the 
primary  current,  it  being  presupposed  that  the  interrupter 
works  the  same  in  all  cases.  The  intensity  of  the  X-rays, 
on  the  contrary,  is  proportional  within  certain  limits  to  the 
strength  of  the  primary  current,  as  the  following  experiment 
shows  :  The  distances  from  the  discharge-apparatus  at  which, 
in  a  certain  case,  the  fluorescence  of  the  barium  platino-cyanide 

34 


RONTGEN    RAYS 

screen  was  just  noticeable  amounted  to  18.1  millimetres,  25.7 
millimetres,  and  37. 5  millimetres,  when  the  strength  of  the 
primary  current  was  increased  from  8  to  16  to  32  amperes. 
The  squares  of  the  distances  are  in  nearly  the  same  ratio  as  the 
corresponding  current-strengths. 

9.  The  results  stated  in  the  last  five  paragraphs  were  derived 
immediately  from  the  individual  experiments  mentioned.  If 
we  review  the  whole  of  these  individual  results,  we  reach  the 
following  conclusions,  being  led  to  them  in  part  by  the  analogy 
which  exists  between  the  behavior  of  optical  rays  and  X-rays  : 

(a)  The  rays  emitted  by  a  discharge  -  apparatus  consist  of  a 
mixture  of  rays  which  are  absorbed  in  different  degrees  and 
which  have  different  intensities. 

(b)  The  composition  of  this  mixture  of  rays  depends  essen- 
tially upon  the  duration  of  the  discharge-current. 

{c)  The  rays  selected  for  absorption  by  various  substances 
are  different  for  the  different  bodies. 

(d)  Since  the  X-rays  are  generated  by  the  cathode  rays,  and 
since  both  have  properties  in  common — production  of  fluores- 
cence, photographic  and  electrical  action,  and  absorbability, 
the  amount  of  which  is  essentially  conditioned  upon  the  density 
of  the  medium  through  which  the  radiation  passes,  etc. — the 
hypothesis  at  once  suggests  itself  that  both  phenomena  are  of 
the  same  nature.  AVithout  wishing  to  bind  myself  uncondi- 
tionally to  this  view,  I  may  remark  that  the  results  of  the  last 
few  paragraphs  are  calculated  to  resolve  a  difficulty  which  has 
existed  in  connection  with  this  hypothesis  up  to  the  present. 
This  difficulty  arises,  first,  from  the  great  difference  between  the 
absorption  of  the  cathode  rays  investigated  by  Herr  Lenard 
and  that  of  the  X-rays  ;  and,  second,  from  the  fact  that  the 
transparency  of  bodies  for  these  cathode  rays  depends  upon  a 
different  law  of  the  densities  of  the  bodies  from  that  govern- 
ing the  transparency  for  the  X-rays. 

As  to  the  first  difficulty,  two  points  should  be  mentioned  : 
(1)  We  have  seen  in  §  7  that  there  are  X-rays  whose  absorp- 
tions are  very  different  ;  and  we  know  from  the  investigations 
of  Hertz  and  Lenard  that  the  different  cathode  rays  also  differ 

35 


MEMOIRS    OX 

from  each  other  with  reference  to  their  ahsorption.  Even  if 
we  admit,  therefore,  that  the  softest  tube  mentioned  on  p.  30 
furnishes  X-rays  whose  absorption  is  far  less  than  that  of  the 
cathode  rays  investigated  by  Ilerr  Lenard,  yet  we  cannot  doubt 
that  there  are  X-rays  which  are  absorbed  more,  and.  on  the 
other  hand,  cathode  rays  which  are  absorbed  less  even  than 
those.  It  therefore  seems  perfectly  possible  that  by  further 
experiments  rays  will  be  found  which  form,  so  far  as  absorp- 
tion is  concerned,  the  link  between  the  one  kind  of  rays  and 
the  other.  (2)  We  found  in  §  4  that  the  specific  transparency  of 
a  body  is  smaller  in  proportion  as  the  plate  traversed  is  thinner. 
Consequently,  if  in  our  experiments  we  had  taken  plates  as 
thin  as  those  of  Herr  Lenard,  we  might  have  obtained  values 
for  the  absorption  of  the  X-rays  which  would  approximate 
more  closely  those  of  Lenard. 

With  reference  to  the  varying  influence  of  the  density  of  bod- 
ies on  their  absorption  of  X-rays  and  of  cathode  rays,  it  should 
be  said  that  this  difference  is  found  to  be  smaller  in  proportion 
as  more  strongly  absorbable  X-rays  are  chosen  for  the  experi- 
ment (§  7  and  §  8),  and  in  proportion  as  the  plates  traversed 
are  made  thinner  (§  5).  Consequenth^  one  must  acknowledge 
the  possibility  that  this  difference  in  the  behavior  of  the  two 
kinds  of  rays  may,  by  means  of  further  experiments,  be  made 
to  vanish  at  the  same  time  as  the  differences  mentioned  above. 

With  reference  to  this  absorbability,  the  rays  which  come 
nearest  to  each  other  are  the  cathode  rays  which  are  especially 
present  in  very  hard  tubes  and  the  X-rays  which  are  emitted 
from  the  platinum  plate  in  very  soft  tubes. 

10.  Besides  exciting  fluorescence,  the  X-rays  have,  as  is  well 
known,  photographic,  electric,  and  other  actions  :  and  it  is  of 
interest  to  know  how  far  these  continue  parallel  with  each 
other  as  the  source  of  radiation  is  altered.  I  have  been  obliged 
to  confine  myself  to  comparing  the  two  actions  first  named. 

The  platinum -aluminium  window  is  suited  for  this  work 
also.  One  of  these  is  placed  upon  a  photographic  plate  which 
is  wrapped  up,  a  second  is  brought  in  front  of  the  fluorescent 
screen,  and  both  are  then  placed  at  equal  distances  from  the 

36 


ROXTGEX    RAYS 

discharge-apparatus.  The  X-rays  had  exactly  the  same  media 
to  traverse  in  order  to  reach  the  sensitive  layer  of  the  photo- 
graphic plate  and  the  barium  platino  -  cyanide.  Daring  the 
exposure  I  observed  the  screen  and  determined  the  window- 
number  ;  after  development,  the  window-number  was  also  de- 
termined upon  the  photographic  plate  ;  and  then  both  num- 
bers were  compared.  The  result  of  these  experiments  is  that, 
using  softer  tubes  (window-numbers  4-T),  no  difference  could  be 
observed  :  but  when  harder  tubes  were  used  it  seemed  to  me 
that  the  window-number  on  the  photographic  plate  was  a  little 
lower,  at  most  one  unit,  than  that  determined  by  means  of  the 
fluorescent  screen.  This  observation,  however,  although  re- 
peatedly confirmed,  is  not  quite  free  from  criticism,  because  the 
determination  of  the  high  window-numbers  at  the  fluorescent 
screen  is  quite  uncertain. 

The  following  result  is,  however,  entirely  certain.  If  we 
arrange,  with  the  photometer  described  in  §  2,  a  hard  and  a 
soft  tube  so  as  to  have  the  same  brightness  at  the  fluorescent 
screen,  and  if  a  photographic  plate  is  substituted  for  the 
screen,  we  see  after  development  of  this  plate  that  the  half  of 
the  plate  which  received  the  rays  from  the  hard  tube  is  con- 
siderably less  blackened  than  the  other.  The  radiations  which 
produce  equal  intensities  of  fluorescence  have  different  photo- 
graphic actions. 

In  considering  this  result  we  must  not  leave  out  of  account 
the  fact  that  neither  the  fluorescent  screen  nor  the  photo- 
graphic plate  uses  up  completely  the  incident  rays ;  both 
transmit  many  rays  which  can  again  produce  fluorescent  or 
photographic  action.  The  result  communicated  holds  true, 
therefore,  only  for  the  thickness  of  the  sensitive  photographic 
film  employed  and  the  layer  of  barium  platino-cyanide  accom- 
panying it. 

How  very  transparent  to  the  X-rays  from  tubes  of  average 
hardness  the  sensitive  film  of  the  photographic  plate  is  is 
shown  by  an  experiment  with  96  ''films "  laid  one  over  another, 
25  centimetres  distant  from  the  source  of  radiation,  exposed 
for  5  minutes,  and  protected  against  the  radiation  of  the  air 

37 


MEMOIRS    ON 

by  an  envelope  of  lead.  Even  on  the  last  one  a  photographic 
action  can  be  recognized  plainly,  while  the  first  is  scarcely 
over-exposed.  Induced  by  this  and  similar  observations,  I  have 
inquired  of  several  firms  who  furnish  photographic  plates 
whether  it  would  not  be  possible  to  prepare  plates  which  were 
more  suited  for  photography  with  X-rays  than  the  ordinary 
ones.     The  samples  forwarded  were  not,  however,  serviceable. 

I  have  had  many  oj^portunities,  as  mentioned  already  on 
p.  31,  to  perceive  that  very  hard  tubes,  under  otherwise  equal 
circumstances,  require  a  longer  time  of  exposure  than  those 
moderately  hard  ;  this  is  easily  understood  if  we  remember  the 
result  communicated  in  §  9,  according  to  which  all  bodies  so 
far  examined  are  more  transparent  for  rays  which  are  emitted 
by  hard  tubes  than  for  those  coming  from  soft  ones.  The  fact 
that  with  very  soft  tubes  the  exposure  must  again  be  long  is 
explained  by  the  lack  of  intensity  of  the  rays  emitted  by  \hem. 

If  the  intensity  of  the  rays  is  increased  by  increasing  the 
primary  current  (see  p.  31),  the  photographic  action  is  in- 
creased in  the  same  degree  as  the  intensity  of  the  fluores- 
cence ;  and  in  this  case,  as  also  in  that  mentioned  above  where 
the  intensity  of  the  radiation  on  the  fluorescent  screen  is  al- 
tered by  changing  the  distance  of  the  screen  from  the  source  of 
the  rays,  the  brightness  of  the  fluorescence  is  proportional,  or 
at  least  nearly  so,  to  the  intensity  of  the  radiation.  This  law 
cannot,  however,  be  applied  generally. 

11.  In  conclusion,  I  beg  the  privilege  of  mentioning  the  fol- 
lowing isolated  points : 

In  a  discharge-tube  properly  made  and  not  too  soft,  the  X- 
rays  come  mainly  from  a  spot  on  the  platinum  plate  struck  by 
the  cathode  rays,  which  is  from  1  to  2  millimetres  in  size.  But 
this  is  not  the  only  starting-point:  the  whole  jilate  and  a  part 
of  the  wall  of  the  tube  emit  rays,  although  to  a  very  small  ox- 
tent.  Cathode  rays  proceed  from  the  cathode  in  all  directions  ; 
their  intensity,  however,  is  important  only  in  the  neighborhood 
of  the  axis  of  the  concave  mirror  ;  and  therefore  the  most  in- 
tense X-rays  originate  on  the  platinum  })late  at  the  point  where 
this  axis  meets  it.     If  the  tube  is  very  hard  and  the  platinum 

33 


RONTGEN    RAYS 

thin,  a  great  many  X-rays  are  emitted  from  the  rear  side  of  the 
platinum  plate,  and,  as  is  shown  by  a  pin-hole  camera,  from  a 
point  which  also  lies  on  the  axis  of  the  mirror. 

In  these  hardest  tubes,  also,  the  maximum  of  intensity  of  the 
cathode  rays  can  be  deflected  from  the  platinum  plate  by  means 
of  a  magnet.  Some  experiments  on  soft  tubes  led  me  to  take 
up  again,  with  better  apparatus,  the  question  of  the  possibility 
of  magnetic  deflection  of  X-rays  ;  I  hope  to  be  able  to  com- 
municate soon  the  results  of  these  experiments. 

The  experiments  mentioned  in  my  first  communication  on 
the  transparency  of  plates  of  the  same  thickness  which  are  cut 
from  a  crystal  according  to  different  directions  have  been  con- 
tinued. I  have  investigated  plates  of  calcite,  quartz,  tourma- 
line, beryl,  aragonite,  apatite,  and  barite.  ^o  influence  of  di- 
rection on  the  transparency  could  be  detected  even  with  the 
improved  apparatus. 

The  fact  observed  by  Herr  Gr.  Brandes,  that  the  X-rays  can 
produce  a  light-sensation  in  the  retina  of  the  eye,  I  have  found 
confirmed.  There  stands  also  in  my  observation-] ournal  a  note 
at  the  beginning  of  the  month  of  November,  1895,  according  to 
which  I  perceived  a  feeble  light-sensation,  which  spread  over 
the  whole  field  of  vision,  when  I  was  in  an  entirely  darkened 
room  near  a  wooden  door  on  the  other  side  of  which  there 
was  a  Hittorf  tube,  whenever  discharges  were  sent  through  the 
tube.  Since  I  observed  this  phenomenon  only  once,  I  thought 
it  a  subjective  one,  and  the  fact  that  I  never  saw  it  repeated  is 
because,  later,  instead  of  the  Hittorf  tube,  other  apparatus  was 
used,  not  exhausted  so  much,  and  not  provided  with  platinum 
anodes.  On  account  of  their  state  of  high  exhaustion,  Hittorf 
tubes  furnish  rays  which  are  only  slightly  absorbed,  and  on  ac- 
count of  the  presence  of  a  platinum  anode,  which  is  struck  by 
the  cathode  rays,  they  furnish  intense  rays,  a  condition  which  is 
favorable  for  the  production  of  the  light-phenomenon  referred 
to.  I  was  obliged  to  replace  the  Hittorf  tubes  by  others,  be- 
cause after  a  very  short  while  all  were  perforated. 

With  the  hard  tubes  now  in  general  use  the  experiment  of 
Brandes  may  be  easily  repeated.     The  following  description  of 


MEMOIRS  ON  RONTGEN  RAYS 

the  mode  of  experimenting  may  be  of  some  interest :  If  a  ver- 
tical metal  slit  some  tenths  of  a  millimetre  broad  is  held  as 
close  as  possible  before  the  open  or  closed  eye^  and  if  the  head, 
completely  enveloped  in  a  black  cloth,  is  then  brought  near  the 
discharge  apparatus,  there  is  observed,  after  some  practice,  a 
weak,  non-uniformly  bright  strip  of  light  which,  according  to 
the  place  where  the  slit  is  in  front  of  the  eye,  takes  a  different 
form — straight,  curved,  or  circular.  By  a  slow  motion  of  the 
slit  in  a  horizontal  direction,  these  different  forms  can  be  made 
to  pass  gradually  from  one  into  the  other.  An  explanation  of 
the  phenomenon  is  found  immediately  if  we  consider  that  the 
ball  of  the  eye  is  cut  by  a  lamellar  sheaf  of  X-rays,  and  if  we 
assume  that  the  X-rays  can  excite  fluorescence  in  the  retina. 

Since  the  beginning  of  my  work  on  X-rays  I  have  tried  re- 
peatedly to  obtain  diffraction  phenomena  with  them ;  several 
times  I  have  obtained  with  narrow  slits,  etc.,  phenomena  whose 
appearance  reminded  one,  it  is  true,  of  diffraction  images  ;  but 
when  by  alteration  of  the  conditions  of  experiment  tests  were 
made  of  the  correctness  of  the  explanation  of  these  images  by 
diffraction,  it  was  refuted  in  every  case  ;  and  often  I  could 
prove  directly  that  the  phenomena  had  arisen  in  a  way  quite 
different  from  diffraction.  I  have  no  experiment  to  describe 
from  which,  with  sufficient  certainty,  I  could  obtain  proof  of  the 
existence  of  diffraction  of  the  X-rays. 

WiJRZBURG,  Phjsikalisclies  Institut  der  Universitiit. 
March  10,  1897. 

WiLiiELM  CoxRAD  RoxTGEN  was  born  March  27,  1845,  in 
Lennep,  Rhine  Province,  Germany,  and  is  at  the  present  time 
Professor  of  Physics  at  the  University  of  Wiirzburg.  He  re- 
ceived his  doctor's  degree  at  Zurich  in  1808,  and  became  then 
an  assistant  to  Kundt  at  Wiirzburg.  He  was  finally  appointed 
Professor  of  Physics  at  Giessen,  from  which  university  he  was 
transferred  to  Wiirzburg.  He  has  been  engaged  in  many  im- 
portant researches  which,  in  the  main,  have  a  bearing  upon 
the  connection  between  electricity  and  ordinary  matter. 

40 


ON  THE  NATURE  OF  THE  EONTGEX  RAYS 

(THE   WILDE   LECTUEE) 

CY 

Sir  G.  G.  STOKES,  Bart.,  M.A.,  LL.D.,  F.R.S. 

(Memoirs  and  Proceedings  of  the  JIanchester  Literary  and  Philosophical 
Society,  41,  Part  1\.,  1896-7.) 


CONTEXTS 

PAGE 

Resume  of  Work  by  Rontgen  and  J.  J.  Thomson 43 

Discussion  of  Properties  of  X-Rays 46 

Discussion  of  Properties  of  Becquerel  Rays 48 

Discussion  of  Properties  of  Cathode  Rays 49 

Propagation  of  Pulses 54 

Theory  of  Rontgen  Rays 55 

Theory  of  Ordinary  Refraction 58 

Theory  of  Becquerel  Rays 62 

Diffraction  of  Pulses 63 


ON  THE  NATURE  OF  THE  RONTGEN  RAYS 

(the  WILDE  LECTURE) 
BY 

Sir  G.  G.  STOKES,  Bart.,  M.A.,  LL.D.,  F.R.S. 

Delivered  July  25,  1S97. 

Ever  since  the  remarkable  discovery  of  Professor  Rontgeu 
was  published,  the  subject  has  attracted  a  great  deal  of  atten- 
tion in  all  civilized  countries,  and  numbers  of  physicists  have 
worked  experimentally,  endeavoring  to  make  out  the  laws  of 
these  rays,  to  determine  their  nature,  if  possible,  and  to  ar- 
range for  their  application.  I  am  sorry  to  say  that  I  have  not 
myself  worked  experimentally  at  the  subject ;  and  that  being 
the  case,  there  is  a  certain  amount  of  presumption,  perhaps,  in 
my  venturing  to  lecture  on  it.  Still,  I  have  followed  pretty 
well  what  has  been  done  by  others,  and  the  subject  borders 
very  closely  on  one  to  which  I  have  paid  considerable  attention ; 
that  is,  the  subject  of  light. 

In  Rontgen's  original  paper  he  stated  that  it  was  shown  ex- 
perimentally that  the  seat  of  these  remarkable  rays  was  the 
place  where  the  so-called  cathodic  rays  fall  on  the  opposite  wall 
of  the  highly  exhausted  tube  in  which  they  are  produced.  I 
will  not  stop  to  describe  what  is  meant  by  cathodic  rays.  It 
would  take  me  too  much  away  from  my  subject,  and  I  may 
assume,  I  think,  that  the  audience  I  am  now  addressing  know 
what  is  meant  by  that  term.  This  statement  of  Rontgen's  was 
not,  I  think,  universally  accepted.  Some  experimentalists  set 
themselves  to  investigate  the  point  by  observing  the  positions 
of  the  shadows  cast  by  bodies  subjected  to  the  discharge  of  the 
Rontgen  rays — to  investigate,  I  say,  the  place  within  the  tube 
from  which  the  rays  appeared  to  come.     Now,  when  the  shad- 

43 


MEMOIRS    Ox\ 

ows  were  received  on  a  photographic  pldte,  and  the  shadow  was 
joined  to  the  snbstance  casting  the  shadow,  and  the  joining 
lines  were  produced  backwards,  as  a  rule  they  tended  more  or 
less  nearly  to  meet  somewhere  witliin  tlie  tube — Crookes'  tube, 
I  will  now  call  it — and  some  people  seem  to  have  had  the  idea 
that  at  that  point  of  meeting  or  approximate  meeting  there 
was  something  going  on  which  was  the  source  of  these  rays.  I 
have  in  my  hands  a  paper  published  in  St.  Petersburg  by  Prince 
B.  Galitzin  and  A.  v.  Karnojitzky,  which  contains  some  very 
elaborate  photographs  obtained  in  this  way.  A  board  was  taken 
and  ruled  with  cross  lines  at  equal  intervals,  and  at  the  points 
of  intersection  nails  were  struck  in  in  an  upright  position. 
The  board  was  placed  on  top  of  the  photographic  plate,  with 
an  opaque  substance  between — a  substance  which  these  strange 
Rontgen  rays  are  capable  of  passing  through,  though  it  is 
impervious  to  light.  The  shadows  cast  by  the  nails  w^re  ob- 
tained on  the  photograph,  and  this  paper  contains  a  number 
of  the  photographs.  It  is  remarkable,  considering  the  some- 
what large  space  in  the  tube  over  which  the  discharge  from 
the  cathode  is  spread,  that  the  shadows  are  as  sharp  as  they 
actually  are ;  and  the  same  thing  may  be  affirmed  of  the  or- 
dinary shadows  of  the  bones  of  the  hand,  for  instance,  which 
one  so  frequently  sees  now.  Another  remarkable  point  in 
these  photographs  is  that  in  some  cases  it  appears  as  if 
there  were  two  shadows  of  the  same  nail,  as  though  there 
were  two  different  sources  from  which  these  strange  rays 
come,  both  situated  within  the  Crookes'  tube.  Now,  have 
we  a  right  to  suppose  that  the  place  of  meeting  of  the  lines 
by  which  the  shadows  are  formed,  prolonged  backwards  into 
the  tube,  is  the  place  which  is  the  seat  of  action  of  these 
rays  ?  I  think  we  have  not.  If  a  portion  of  the  Crookes' 
tube  which  is  influenced  by  the  cathode  discharge  be  isolat- 
ed by,  we  will  say,  a  lead  screen  containing  a  small  hole,  you 
get  a  portion  of  the  cathodic  rays  which  come  out  through 
that  small  hole,  and  you  can  trace  what  becomes  of  them  be- 
yond. It  is  found  that  the  influence  is  decidedly  stronger 
in  a  normal  direction  than  in  oblique  directions.      Professor 

44 


RONTGEN    RAYS 

J.  J.  Thomson,  of  Cambridge,  who  has  worked  a  great  deal 
experimentally  at  this  subject,  mentioned  that  to  me  as  a 
striking  thing.  You  might  imagine  that  the  fact  that  the 
shadows  appear  to  be  cast  approximately  from  a  source  within 
the  tube  could  be  accounted  for  in  this  way.  Supposing,  as 
Rontgen  believed,  that  the  seat  of  the  rays  is  in  the  place 
where  the  cathode  discharge  falls  on  the  surface  of  the  glass, 
those  which  come  in  an  oblique  direction  have  to  pass  through 
a  greater  thickness  of  glass  than  those  which  come  in  a  nor- 
mal direction.  Xow,  glass  is  only  partially  transparent  to 
the  Rontgen  rays  ;  therefore  the  oblique  rays  would  be  more 
absorbed  in  passing  through  the  glass  than  the  rays  which 
come  in  a  normal  direction.  I  mentioned  that  to  Professor 
Thomson,  but  he  said  he  thought  the  difference  between  the 
intensity  of  the  rays  which  come  out  obliquely  and  those 
which  come  out  in  a  normal  direction  was  much  too  great 
to  be  accounted  for  in  that  way.*  I  will  take  it  as  a  fact, 
without  entering  at  present  into  any  speculation  as  to  the 
reason  for  it,  that  the  Rontgen  rays  do  come  out  from  the 
glass  wall  more  copiously  in  a  normal  direction  than  in  an 
oblique   direction.     Assuming   this,  we   can   rightly  say  that 

*  I  have  found  b}^  subsequent  inquiry  that  the  experiment  referred  to 
was  not  made  by  Professor  Thomson  himself,  but  by  Mr.  C.  M.  McClel- 
land, in  the  Cavendish  Laboratory,  and  that  on  being  recently  repeated 
with  the  same  tube  the  efTect  of  the  X-rays  was  found  to  be  by  no  means 
so  much  concentrated  towards  the  normal  to  the  wall  of  the  tube  as  in  the 
former  experiment.  It  seems  likely  that  the  difference  may  have  been  due 
to  use  of  the  tube  in  the  interval,  which  would  have  made  the  exhaustion 
higher,  and  caused  the  X-rays  given  out  to  be  of  higher  penetrative  power, 
so  as  to  render  the  increased  thickness  of  glass  which  the  rays  emerging 
obliquely  had  to  pass  through  to  be  of  less  consequence.  But  the  subject 
is  still  under  examination.  In  consequence  of  the  result  obtained  in  the 
second  experiment,  the  statement  in  the  text  should  be  less  absolute  ;  but 
it  may  very  well  have  happened  that  in  the  experiments  of  others  the  con- 
ditions may  more  nearly  have  agreed  with  those  of  the  first  experiment, 
causing  w^hat  we  may  call  the  resultant  activity  of  the  X-rays  to  have  had 
a  direction  leaning  towards  the  normal  drawn  from  the  point  casting  the 
shadow  to  the  wall  of  the  tube. 

4o 


MEMOIRS    ON 

the  results  obtained  by  Prince  Galitzin  and  M.  v.  Karnojit- 
zky,  and  similar  results  obtained  by  others,  do  not  by  any 
means  prove  that  the  seat  of  the  rays  is  within  the  tube. 
Suppose,  for  example,  that  the  tube  were  spherical,  and  a  por- 
tion of  this  spherical  surface  were  reached  by  the  cathodic 
rays;  if  the  Rontgen  rays  which  passed  outside  came  wholly, 
we  will  say,  in  a  normal  direction,  produce  the  directions 
backwards  and  you  will  get  the  centre  of  the  tube.  But  we 
have  no  right  to  say  from  that  that  there  is  anything  particular 
going  on  in  the  centre  of  the  spherical  tube.  The  result  is 
perfectly  compatible  with  Rontgen's  original  assertion,  which  I 
believe  to  be  true,  as  to  the  seat  of  the  rays. 

Everything  tends  to  show  that  these  Rontgen  rays  are  some- 
thing which,  like  rays  of  light,  are  propagated  in  the  ether. 
What,  then,  is  the  nature  of  this  process  going  on  in  the  ether  ? 
Some  of  the  properties  of  the  Rontgen  rays  are  very  surprising, 
and  very  unlike  what  we  are  in  the  habit  of  considering  with 
regard  to  rays  of  light.  One  of  the  most  striking  things  is  the 
facility  with  which  they  go  through  bodies  which  are  utterly 
opaque  to  light,  such,  for  example,  as  black  paper,  board,  and 
so  forth.  If  that  stood  alone  it  would,  perhaps,  not  constitute 
a  very  important  difference  between  them  and  light.  A  red 
glass  will  stop  green  rays  and  let  red  rays  through  ;  and  just  in 
the  same  way  if  the  Rontgen  rays  were  of  the  nature  of  the  or- 
dinary rays  of  light,  it  is  possible  that  a  substance,  although 
opaque  to  light,  might  be  transparent  to  them.  So,  as  I  say, 
that  remarkable  property,  if  it  stood  alone,  would  not  neces- 
sarily constitute  any  great  difference  of  nature  between  them 
and  ordinary  light.  But  there  are  other  properties  which  are 
far  more  difficult  to  reconcile  with  the  idea  that  the  Rontgen 
rays  are  of  the  nature  of  light.  There  is  the  absence,  or  almost 
complete  absence,  of  refraction  and  reflection.  Another  re- 
markable property  of  these  rays  is  the  extreme  sharpness  of  the 
shadows  which  they  cast  when  the  source  of  the  rays  is  made 
sufficiently  narrow.  The  shadows  are  far  sharper  than  those 
produced  under  similar  circumstances  by  light,  because  in  the 
case  of  light  the  shadows  are  enlarged  as  the  effect  of  diffrac- 

46 


ROXTGEX    RAYS 

tiou.  This  absence,  or  almost  complete  absence,  of  diffraction 
is  then  another  circumstance  distinguishing  these  rays  from  or- 
dinary rays  of  light.  In  face  of  these  remarkable  differences, 
those  who  speculated  with  regard  to  the  nature  of  the  rays  were 
naturally  disposed  to  look  in  a  direction  in  which  there  was 
some  distinct  difference  from  the  j^rocess  which  we  conceive  to 
go  on  in  the  propagation  and  production  of  ordinary  rays  of 
light.  Those  who  have  speculated  on  the  dynamical  theory  of 
double  refraction  have  been  led  to  imagine  the  possible  exist- 
ence in  the  ether  of  longitudinal  vibrations,  as  well  as  those 
transversal  vibrations  which  we  know  to  constitute  light.  If 
we  were  to  suppose  that  the  Rontgen  rays  are  due  to  longitudi- 
nal vibrations,  that  would  constitute  such  a  very  great  difference 
of  nature  between  them  and  rays  of  light  that  a  very  great  dif- 
ference in  properties  might  reasonably  be  expected.  But  as- 
suming that  the  Rontgen  rays  are  a  process  which  goes  on  in 
the  ether,  are  the  vibrations  belonging  to  them  normal  or  trans- 
versal ?  If  we  could  obtain  evidence  of  the  polarization  of 
those  rays,  that  would  prove  that  the  vibrations  were  not  nor- 
mal, but  transversal.  But  if  we  fail  to  obtain  evidence  of  polar- 
ization, that  does  not  at  once  prove  that  the  vibrations  may  not 
after  all  be  transversal,  because  the  properties  of  these  rays  are 
such  as  to  lead  us  dpiiori  to  expect  great  difficulties  in  the  way 
of  putting  in  evidence  their  polarization,  if,  indeed,  they  are 
capable  of  polarization  at  all.  Several  experimentalists  have 
attempted,  by  means  of  tourmalines,  to  obtain  evidence  of 
polarization,  but  the  result  in  general  has  been  negative.  Of 
the  two  photographic  markings  that  ought  to  be  of  unequal 
intensity  on  the  supposition  of  polarization,  one  could  not  say 
with  certainty  that  one  was  darker  than  the  other.  Another 
way  of  obtaining  polarized  light  is  by  reflection  at  the  proper 
angle  from  glass  or  other  substance  ;  but,  unfortunately  for  the 
success  of  such  a  method,  the  Rontgen  rays  refuse  to  be  regu- 
larly reflected,  except  to  a  very  small  extent  indeed.  The  au- 
thors of  the  paper  to  which  I  have  already  referred  appear  to 
have  had  some  success  with  the  tourmaline.  Like  others  who 
have  worked  at  the  same  experiment,  they  took  a  tourmaline 

47 


MEMOIRS    ON 

cut  parallel  to  the  axis  and  put  on  top  of  it  two  othere,  also  cut 
parallel  to  the  axis,  and  of  equal  thickness,,  which  were  placed 
with  their  axes  parallel  and  perpendicular  respectively  to  that 
of  the  under  tourmaline.  But  they  supplemented  this  method 
bv  a  device  which  is  not  explained  in  the  paper  itself,  although 
a  memoir  is  referred  to  in  which  the  explanation  is  to  be  found 
— at  least  by  those  who  can  read  the  Bussian  language,  which, 
unfortunatelv,  I  cannot.  I  can,  therefore,  only  guess  what  the 
method  was.  It  is  something  depending  on  the  superposition  of 
sensitive  photographic  films.  I  suspect  they  had  several  photo- 
graphic films  superposed,  took  the  photographs  on  these,  and 
then  took  them  asunder  for  development,  and  after  develop- 
ment put  them  together  again  as  they  had  been  originaUy. 
They  consider  that  they  have  succeeded  in  obtaining  evidence 
of  a  certain  amount  of  polarization.  If  we  assume  that  evi- 
dence to  be  undoubted,  it  decides  the  question  at  once.  But 
as  the  experiment,  as  made  in  this  way,  is  rather  a  delicate 
one.  it  is  important  for  the  evidence  that  we  should  consider  as 
well  what  we  may  call  the  Becquerel  rays.  If  time  permits,  I 
shall  have  something  to  say  abont  these  towards  the  close  of  my 
lecture,  but,  for  the  present,  I  shall  say  merely  that  they  ap- 
pear to  be  intermediate  in  their  properties  between  the  Bontgen 
rays  and  rays  of  ordinary  light.  The  Becquerel  rays  undoubt- 
edly admit  of  polarization,  and  the  evidence  appears  on  the 
whole  pretty  conclusive  that  the  Bontgen  rays,  like  rays  of  or- 
dinary light,  are  due  to  transversal,  and  not  to  longitudinal, 
vibrations.  It  remains  to  be  explained,  if  we  can  explain  it, 
wherein  lies  the  difference  between  the  nature  of  the  Bontgen 
rays  and  rays  of  ordinary  light  which  accounts  for  the  strange 
and  remarkable  difference  in  the  properties  of  the  two.  I  may 
mention  that,  although  Cauchy  and  Xeumann.  and  some  others 
who  have  written  on  the  dynamical  theory  of  double  refraction, 
have  been  led  to  the  contemplation  of  normal  vibrations.  Green 
has  put  forward  what  seems  to  me  a  very  strong  argument 
against  the  existence  of  normal  vibrations  in  the  case  of  light. 
The  argument  Green  used  always  weighed  strongly  with  me 
against  the  supposition   that  the  Bontgen   rays  were  doe  to 

48 


RONTGEX    RAYS 

longitnclinal  vibrations ;  and  the  experiments  bj  which,  as  I 
conceive,,  the  possibility  of  their  polarization  has  now  been 
established  go  completely  in  the  same  direction,  showing  that 
they  are  due,  assuming  them  to  be  some  process  going  on  in 
the  ether,  to  a  transversal  disturbance  of  some  kind. 

Xow,  the  so  -  called  cathodic  rays  are,  as  we  may  say,  the 
parents  of  the  Rontgen  rays.  Consequently,  if  we  are  to  ex- 
plain the  nature  of  the  Rontgen  rays,  it  is  very  important  that 
we  should  have  as  clear  ideas  as  may  be  permissible  of  the  nat- 
ure of  the  cathodic  rays.  Xow,  two  views  have  been  enter- 
tained as  to  the  nature  of  the  cathodic  rays.  According  to  one 
view,  they  are  not  rays  of  light  at  all,  but  streams  of  molecules 
which  are  projected  from  the  cathode,  and,  if  the  exhaustion 
within  the  tube  be  sufficient,  reach  the  opposite  wall.  That 
was  the  idea  under  which  Crookes  worked  in  his  well-known 
experiments,  and,  so  far  as  I  know,  it  is  the  view  held  by  all 
physicists  in  this  country.  Another  opinion,  however,  has 
been  published,  and  there  are  some  eminent  physicists  who 
favor  it,  especially,  I  think,  in  Germany.  According  to  this 
latter  opinion,  the  cathodic  rays  are,  like  rays  of  light,  some 
process  going  on  in  the  ether.  The  cathodic  ray,  coming  from 
the  cathode  towards  the  opposite  wall  of  the  tube,  is  invisible 
as  such  if  you  look  across  it.  There  is  in  reality  a  faint  blue 
light  ordinarily,  but  not  necessarily,  seen  when  you  look  across 
it.  Lenard,  in  his  most  elaborate  and  remarkable  experiments, 
succeeded  in  producing  the  cathodic  rays  within  a  space  from 
which  the  gas  was  so  very  nearly  completely  taken  away 
that,  although  the  cathodic  rays  passed  freely  through  the 
space,  there  was  no  appearance  of  the  blue  light  when  you 
viewed  their  path  transversely.  They  produced,  however, 
the  ordinary  effect  of  phosphorescence  at  the  other  end  of 
the  tube.  The  appearance,  then,  may  be  analogous  to  that  of 
a  sunbeam  coming  from  a  hole  in  the  clouds.  If  it  were  not 
for  the  slight  amount  of  dust  and  suspended  matter  in  the  air, 
the  sunbeam  would  be  invisible  if  you  looked  across  it.  But  as 
the  air  is  never  free  from  motes,  you  see  the  path  of  the  sun- 
beam vrhen  you  look  across  it  by  the  light  reflected  from  these 
D  49 


MEMOIRS    OX 

motes.  Something  of  the  same  kind  may  be  conceived  to  take 
place  with  regard  to  the  cathodic  rays  if  they  are  some  process 
going  on  in  the  ether.  But  there  are  very  great  difficulties  in 
the  way  of  this  second  hypothesis,  and  especially  as  regards 
certain  properties  of  the  cathodic  rays.  In  the  first  place,  they 
act  mechanically.  In  Crookes'  experiments  he  succeeded  in 
causing  a  light  windmill,  if  I  may  so  describe  it,  to  spin  rapidly 
under  the  action  of  the  rays.  And  when  they  were  received 
on  a  very  thin  film  of  blown  glass,  the  glass  was  actually  bent 
under  them  as  they  fell  upon  it.  But  that  is  not  all.  These 
cathodic  rays  appear  to  proceed  in  a  normal  direction  from  the 
cathode,  and  ordinarily  proceed  in  straight  lines.  But — and 
this  is  the  important  point — they  are  capable  of  being  deflected 
in  their  path  both  by  electro-static  force  and  by  magnetic  or 
electro-dynamic  force.  Xothing  whatever  of  the  kind  occurs 
with  rays  of  light,  and  there  are  enormous,  almost  insuj^erable, 
difficulties  in  the  supposition  of  any  such  deflection  occurring 
if  the  cathodic  rays  are  a  process  going  on  in  the  ether.  I  will 
not  go  into  all  the  arguments  for  and  against  the  two  views, 
especially  as  the  cathodic  rays  only  enter  incidentally  into  the 
subject  I  have  chosen  to  bring  before  you.  I  will  confine  my- 
self to  one  or  two  of  the  chief  difficulties  in  the  way  of  the  sup- 
position that  the  cathodic  rays  are  streams  of  molecules.  In 
his  admirable  experiments  Lenard  produced  the  cathodic  rays 
in  a  tube  which  was  highly  exhausted,  but  not  exhausted  to 
the  very  highest  degree  that  art  can  obtain.  When  you  get  to 
such  tremendous  exhaustions  as  that  you  cannot  get  the  dis- 
charge to  pass  through  the  tube.  "What  did  he  do  ?  Previous 
experiments  had  shown  that  certain  metals — aluminium  espe- 
cially— are^  or  appear  to  be,  to  a  certain  extent  transparent  to 
these  rays.  AVorking  on  the  supposition  that  an  aluminium 
plate  is,  to  a  certain  extent,  transparent  to  these  rays,  Lenard 
constructed  a  tube,  highly  exhausted,  but  not  to  the  very  last 
degree.  .Then  a  window  of  aluminium-foil — a  very  small  aper- 
ture for  mechanical  reasons — was  fastened  in  an  air-tight  man- 
ner at  the  end  of  the  tube,  to  lead  into  a  second  tube  provided 
with  a  phosphorescent  screen.     The  cathodic  rays  produced  in 

50 


RONTGEX    RAYS 

the  first  tube  fell  upon  the  aluminium  plate  and,  as  Lenard 
su23posed,  passed  through  it  as  rays  of  light  would  pass  through 
glass.  And  so  he  got  them  into  the  second  tube,  and,  it  not 
being  necessary  to  make  an  electric  discharge  ^^ass  through  the 
second  tube,  he  could  exhaust  it  to  the  very  highest  power  of 
skill  that  he  had.  It  was  a  work  of  days  and  days.  The  ca- 
thodic  rays  behaved  in  this  very  highly  exhausted  tube  like  ordi- 
nary cathodic  rays.  We  are  asked  to  assume  that  we  are  dealing 
here  with  a  vacuum,  and  according  to  Lenard  that  shows — and 
no  doubt  it  would  if  we  grant  the  assumption — that  it  is  no 
longer  a  question  of  matter,  but  of  some  process  going  on  in 
the  ether.*  And,  apparently  on  the  strength  of  that  very 
elaborate  experiment,  Rontgen  in  his  first  paper  seems  to  have 
been  of  the  opinion  that  the  cathodic  rays  were  something  go- 
ing on  in  the  ether.  But  are  we  justified  in  assuming  that  we 
are  here  dealing  with  a  perfect  vacuum  ?  I  do  not  think  we 
are.  I  believe  it  passes  the  power  of  art  to  produce  a  perfect 
vacuum.  You  always  have  a  little  residue  of  which  yott  cannot 
absolutely  get  rid,  and  some  of  Lenard's  own  figures  show  the 
effect  of  the  residual  gas.  He  isolated  by  screens  a  small  part 
of  the  cathodic  discharge  in  the  second  tube,  and  received  it 
on  a  phosphorescent  screen.  He  represents  the  phosphores- 
cent light  in  the  tube  as  consisting  of  a  bright  nucleus  sur- 
rounded by  a  less  bright  halo.  The  bright  nucleus  was  such  as 
would  be  produced  if  the  cathodic  rays  were  rays  of  light,  pro- 
vided that  that  light  were  incapable  of  diffraction.  But,  then, 
how  do  you  account  for  the  halo  ?  The  blue  light  by  which 
the  cathodic  rays  are  seen  under  ordinary  circumstances  is  due, 
I  believe,  to  an  interference  of  the  projected  molecules  with 
the  molecules  of  the  gas.  In  some  of  Lenard^s  experiments  he 
received  the  cathodic  rays  in  the  first  tube  into  the  air,  and  a 
considerable  amount  of  this  blue  light  was  seen.  The  appear- 
ance was  much  as  if  you  had  admitted  a  beam  of  light  into  a 

*  Even  if  the  vacuum  were  perfect,  and  the  result  were  still  the  same, 
that  would  uot  disprove  the  theory  that  the  cathodic  rays  are  streams  of 
molecules,  for  the  molecules  might  have  been  obtained  from  the  alumin- 
ium window  itself. 

51 


MEMOIRS    ON 

mixture  of  milk  and  water.  To  my  mind  this  fainter  halo  in 
the  most  refined  of  Lenard's  experiments,  lying  outside  this 
well-defined  nucleus,  was  evidence  that  the  vacuum,  in  spite 
of  all  the  skill  and  time  expended  upon  it,  was  not  perfect. 
And  for  aught  we  know  to  the  contrary — I  believe,  indeed,  it 
is  the  case — the  cathodic  rays  in  the  second  highly  exhausted 
tube  were  really  streams  of  molecules  coming  from  the  residual 
gas  in  the  tube.  But  now  comes  a  difficulty  with  regard  to  the 
passage  of  the  cathodic  rays  through  an  aluminium  plate.  If 
the  cathodic  rays  were  something  going  on  in  the  ether  we  might 
very  well  understand  that  an  aluminium  plate  might  be  trans- 
parent to  them  although  opaque  to  ordinary  rays  of  light.  But 
if  the  cathodic  rays  are  really  streams  of  molecules,  how  can 
we  imagine  that  they  get  through  the  plate  ?  Do  they  get 
through  the  plate  ?  I  do  not  believe  they  do.  Do  they  riddle 
the  plate  like  a  bullet  going  through  a  thin  piece  of  board  ?  I 
do  not  think  it.  Suppose  you  have  a  trough  containing  a  solu- 
tion of  sulphate  of  copper,  and  at  the  ends  of  it  you  have  two 
copper  plates ;  if  you  send  an  electric  current  through  the 
trough,  copper  is  eaten  away  at  the  anode  and  deposited  at  the 
cathode.  Now,  suppose  you  divide  this  trough  into  two  by  a 
plate  of  copper,  you  still  have  copper  eaten  away  at  the  original 
anode  and  copper  deposited  at  the  original  cathode.  The  in- 
terposed plate  really  divides  the  cell  into  two,  in  each  of  which 
electrolysis  goes  on,  so  that  you  have  not  only  copper  eaten 
away  at  one  end  of  the  trough  and  deposited  at  the  other,  but 
in  your  interposed  plate  you  have  copper  eaten  away  at  one 
side  and  deposited  at  the  other.  So  it  may  be  that  the  second 
surface  of  the.  aluminium-foil  becomes,  as  it  were,  a  new  cath- 
ode, and  starts  cathodic  rays.  This,  perhaps,  is  not  what  we 
should  have  anticipated  beforehand.  Still,  there  is  nothing 
unnatural  in  it,  and  nothing,  it  seems  to  me,  in  consequenQ.e 
of  which  you  would  be  obliged  to  reject  the  theory  which 
makes  the  cathodic  rays  to  be  streams  of  molecules.  There 
are  one  or  two  other  difficulties  mentioned  by  Wiedemann,  but 
I  do  not  think  they  are  at  all  serious ;  they  are  certainly  not 
so  serious  as  the  one  I  have  just  referred  to.     I  will,  therefore, 

53 


ROXTGEN    RAYS 

pass  on.  The  possibility  of  deflecting  the  cathodic  rays  by 
electrostatic  and  magnetic  forces  seems  to  be  an  insuperable 
difficulty  in  the  way  of  the  theory  which  makes  them  to  be  a 
process  going  on  in  the  ether;  but  both  of  these  are  perfectly 
in  accordance  with  what  was  to  be  expected  on  the  supposition 
that  they  are  streams  of  molecules,  provided  you  remember 
that  these  molecules  are  highly  charged  with  electricity.  A 
moving  charged  body  behaves  as  regards  deflection  like  an 
electric  current.  Again,  if  you  have  highly  charged  molecules 
in  the  neighborhood  of  a  positively  or  negatively  statically 
charged  body,  they  will  be  attracted  or  repelled,  and  the  de- 
flections of  the  rays  are  precisely  what  was  to  be  expected  ac- 
cording to  that  theory.  I  think  we  may  assume  that  the 
cathodic  rays  are  really  streams  of  electrified  molecules  which 
strike  against  the  opposite  wall  of  the  tube,  or,  as  I  will  now 
call  it,  the  target.  Now,  when  a  molecule,  coming  in  this 
way  from  the  cathode,  strikes  the  target,  how  does  the  mole- 
cule act  ?  It  may  act  in  two  ways.  It  may  act  as  a  mass  of 
matter,  infinitesimal  though  it  be,  by  virtue  of  its  momentum 
— by  virtue  of  its  mass  and  velocit}^ — and  it  may  act  also  as  a 
charged  body,  a  statically  charged  body.  What  the  appropri- 
ate physical  idea  is  of  a  statically  charged  body  is  more  than  I 
can  tell  you.  I  was  talking  not  long  ago  to  Lord  Kelvin  about 
it — and  he  is  a  far  higher  authority  in  electrical  matters  than  I 
am  —  and  he  considers  that  the  physical  idea  of  a  statically 
charged  body  is  still  a  mystery  to  us.  Well,  if  these  charged 
molecules  strike  the  target  we  may  think  it  exceedingly  prob- 
able that  by  virtue  of  their  charge  they  produce  some  sort  of 
disturbance  in  the  ether.  This  disturbance  in  the  ether  would 
spread  in  all  directions  from  the  place  of  disturbance,  so  that 
each  projected  molecule  would  on  that  supposition  become,  on 
reaching  the  target,  a  source  of  ethereal  disturbance  spread- 
ing in  all  directions.  Well,  what  is  the  character  of  such  a 
disturbance  ?  The  problem  of  diffraction,  dynamically  consid- 
ered, may  be  supposed  to  reduce  itself  to  this.  Suppose  you 
have  an  infinite  mass  of  an  elastic  medium,  and  suppose  a 
small  portion  is  disturbed  in  the  most  general  way  possible, 

53 


MEMOIRS    ON 

what  will  take  place  ?  A  wave  of  disturbance  will  spread  out 
spherically  from  the  place  of  disturbance.*  You  might  at  first 
sight  suppose  that  you  could  have  a  wave,  in  any  limited  region 
of  which  you  might  have  a  transversal  disturbance  in  some  one 
direction,  the  same  all  through  the  thickness  of  the  shell  oc- 
cupied by  the  wave,  though  naturally  the  direction  of  disturb- 
ance might  vary  from  one  region  to  another  more  or  less  dis- 
tant region.  But  the  dynamical  theory  shows  that  that  is  not 
possible.  In  any  limited  region,  or  elementary  area,  as  we  may 
regard  it,  of  the  wave,  as  you  pass  in  a  direction  perpendicular 
to  the  front,  the  disturbance  in  one  direction  must  be  ex- 
changed for  a  disturbance  in  the  opposite  direction,  in  such  a 
manner  that  ultimately — that  is,  when  the  radius  of  the  wave 
is  very  large  compared  with  its  thickness — the  integral  of  the 
disturbance  in  one  direction,  which  we  may  designate  as  posi- 
tive, must  be  balanced  by  the  integral  of  the  disturbance  in 
the  opposite,  or  negative,  direction.  The  simplest  sort  of 
'^  pulse,"  as  I  will  call  it,  in  order  to  distinguish  it  from  a  peri- 
odic undulation,  would  be  one  consisting  of  two  halves  in 
which  the  disturbances  were  in  opposite  directions.  The  pos- 
itive and  negative  parts  are  not  necessarily  alike,  as  one  may 
make  up  by  a  greater  width,  measured  in  the  direction  of 
propagation,  for  a  smaller  amplitude  ;  but  it  will  be  simplest 
to  think  of  them  as  alike,  except  as  to  sign.  The  following 
figure  represents  this  conception,  the 
positive  and  negative  halves  being  dis- 
tinguished by  a  difference  of  shading. 
According  to  the  view  here  put  forward,  the  Rontgen 
emanation  consists  of  a  vast  succession  of  independent  pulses, 
starting  respectively  from  the  points  and  at  the  times  at 
which  the  individual  charged  molecules  projected  from  the 
cathode  impinge  on  the  target.  At  first  sight  it  might  ap- 
pear as  if  mere  pulses  would   be   inadequate   to  account  for 

*  If  tlie  medium  be  compressible  there  will  be  two  waves,  that  which 
travels  the  more  swiftly  consisting  of  normal  vibrations  ;  but  the  opinion 
has  already  been  expressed  that  it  is  transversal  vibrations  with  which  we 
are  concerned. 

54 


ROXTGEN    RAYS 

the  effects  produced,  seeing  that  in  the  case  of  light  we 
have  to  deal  with  series  consisting  each  of  a  ver}^  great 
number  of  consecutive  undulations.  But  we  must  bear  in 
mind  how  vast,  according  to  our  theoretical  views,  must  be 
the  number  of  molecules  contained  in  the  smallest  quantity 
of  ponderable  matter  of  which  we  can  take  cognizance  by  our 
senses.  Hence,  small  as  is  the  quantity  of  matter  projected 
in  a  given  short  time  from  the  cathode,  it  may  yet  be  suffi- 
cient to  give  rise  to  pulses  the  number  of  which  is  inconceiv- 
ably great.  It  remains  to  consider  in  what  way  this  concep- 
tion may  enable  us  to  explain  the  most  striking  properties  of 
the  Eontgen  rays  in  relation  to  the  contrasts  which  they  offer 
to  rays  of  light. 

The  most  elementary  difference,  as  being  one  which  has  re- 
lation only  to  propagation  in  the  ether,  consists  in  the  absence, 
or,  at  any  rate,  almost  complete  absence,  of  diffraction.  As 
the  different  pulses  are  by  hypothesis  quite  independent  of 
one  another,  we  have  to  exj^lain  this  phenomenon  for  a  single 
pulse. 

In  the  figure  let  CB  be  a  portion  of  a  spherical  pulse  spread- 
ing outwards  from  the  centre  of  disturbance  (which  I  will  call 
0)  from  which  it  came,  P  a  point  in 
front  of  the  wave,  where  the  disturb- 
ance which  will  arrive  there  is  sought. 
From  P  let  fall  a  normal  PQ  on  the 
front  of  the  wave,  and  let  AB,  taken 
around  Q,  be  a  small  portion  of  the 
spherical  shell  which  at  the  present 
moment  is  the  seat  of  the  pulse,  and  suppose  the  breadth  of 
AB  to  be  small  compared  with  PQ  and  with  the  radius  of 
the  shell,  but  large  compared  with  the  shelFs  thickness.  Let 
CD  be  an  element  of  the  shell  of  similar  size  to  AB,  but  sit- 
uated in  a  direction  from  P  distinctly  inclined  to  PQ  ;  and 
supposing  all  the  disturbance  in  the  shell  stopped  except  what 
occupies  one  or  other  of  the  elements  AB,  CD,  let  us  inquire 
what  will  be  the  disturbance  subsequently  produced  at  P  in 
the  two  cases  respectively. 

00 


MEMOIRS    OX 

I  have  shown  elsewhere*  that  in  onr  present  problem  the 
disturbance  at  P  is  expressed  by  a  double  integral  taken  over 
such  portion  of  the  surface  of  a  sphere  with  P  for  centre  and 
U  for  radius  {b  being  the  velocity  of  propagation)  as  lies  with- 
in the  disturbed  region,  which  in  this  case  is  the  spherical 
shell  or  a  part  of  it.  It  will  be  convenient  to  think  of  a  series 
of  spheres  drawn  round  P  with  radii  M  for  increasing  values 
of  t.  When  t  is  such  that  the  sphere  just  touches  the  shell  at 
Q,  and  then  goes  on  increasing,  the  disturbance  is  nearly  the 
same  all  over  that  portion  of  the  surface  of  the  sphere  which 
lies  within  the  small  region  AB,  and  that,  whether  we  take 
the  portion  of  the  expression  for  the  disturbance  at  P  which 
depends  on  the  disturbance  (displacement  or  velocity)  at  the 
surface  of  the  sphere  whose  radius  is  bt,  or  the  portion  which 
depends  on  the  differential  coefficient  of  the  displacement  or 
velocity  with  respect  to  a  radius  vector  drawn  from  0.  Con- 
sequently the  positive  and  negative  parts  of  the  disturbance 
will  reach  P  in  succession.  But  if  instead  of  the  small  portion 
AB  of  the  shell  we  take  CD,  lying  in  a  direction  from  P  not 
very  near  the  normal,  it  is  easy  to  see  that  the  positive  and 
negative  parts  of  the  disturbance  expressed  by  our  double  in- 
tegral, reaching  as  they  do  P  simultaneously,  almost  complete- 
ly cancel  each  other.  And  this  cancelling  is  so  much  more 
nearly  complete  as  the  obliquity  is  greater,  and  likewise  as  the 
thickness  of  the  shell  is  smaller.  If,  then,  the  disturbance  in 
the  ether  consequent  on  the  arrival  of  any  projected  molecule 
at  the  target  is  very  prompt,  lasting,  it  may  be,  only  a  very 
small  fraction  of  the  period  of  a  single  vibration  of  the  etlier 
in  the  case  of  light,  our  shell  will  be  so  thin  that  a  small  iso- 
lated portion  of  the  Rontgen  discharge  is  propagated  so  nearly 
wholly  in  the  direction  of  a  normal  to  the  wave  that  the  almost 
complete  absence  of  diffraction  is  thus  accounted  for.  f 

*  "On  the  Dynamical  Theory  of  Dlffraclion,"  Cambridge  Philosophical 
Transactions,  vol.  ix.,  p.  1  ;  or  Collected  Papers,  vol.  ii.,  p.  243,  Arts,  19-22. 

f  It  is  known  that  there  is  a  difference  of  quality  in  Rontgen  rays,  and 
that  the  Rontgen  discharge  may  be  filtered  by  absorption.  It  is  known 
also  that  the  increased  exhaustion  in  a  Crookes'  tube,  which  is  accompanied 

56 


ROXTGEX    RAYS 

The  explanation  which  has  just  been  given  of  the  apparent 
absence  of  diffraction  in  the  case  of  the  Rontgen  rays  is  closel}' 
analogous  to  the  ordinary  explanation  of  the  existence  of  rays 
and  shadows.  It  differs,  however,  in  this  respect,  that  here 
we  are  dealing  with  a  single  pulse,  whereas  in  the  case  of  light 
we  are  dealing  with  an  indefinite  succession  of  disturbances. 
In  order  to  understand  the  sharpness  of  the  shadows  produced 
by  the  Rontgen  rays,  we  are  not  obliged  to  suppose  that  the 
disturbance  is  periodic  at  all.  It  must  be  partly  negative  and 
partly  positive,  and  that  being  the  case,  if  the  thickness  of  the 
shell  is  very  small,  the  amount  of  diffraction  will  be  very  small, 
too.  Those  who  have  attempted  to  obtain  evidence  of  the  dif- 
fraction of  the  Rontgen  rays  have  been  led  to  the  conclusion 
that  if  the  rays  are  periodic  at  all  the  period  is  something  enor- 
mously small — perhaps  thirty  times,  perhaps  a  hundred  times, 
as  small  as  the  wave-length  of  green  light.  It  seems  difficult 
to  imagine  by  what  process  you  could  get  such  very  small  vi- 
brations, if  vibrations  there  be.  It  is  easier  to  understand  how 
the  arrival  of  charged  molecules  at  the  cathode  might  produce 
disturbances  which  are  almost  abrupt. 

Well,  then,  this  is  what  I  conceive  to  constitute  the  Rontgen 
rays.  You  have  a  rain  of  molecules  coming  from  the  electri- 
cally charged  cathode,  which  you  may  think  of  as  the  rain-drops 
in  a  shower.  They  strike  successively  on  the  target,  each  mole- 
cule on  striking  the  target  producing  a  pulse,  as  I  have  called 
it,  in  the  ether,  which  is  essentially  partly  positive  and  partly 
negative  ;  and  you  have  a  vast  succession  of  these  pulses  coming 
from  the  various  points  of  the  target  which  are  not  protected 
by  some  screen  interposed  for  the  purpose  of  experiment. 

by  increasing  difficulty  in  sending  a  discbarge  through  it,  has  the  effect 
of  giving  rise  to  increasing  penetrative  power  in  the  Rontgen  rays  which  it 
gives  out.  It  seems  to  me  probable  that  this  difference  of  qualify  corre- 
sponds to  a  more  or  less  close  approach  to  perfect  abruptness  in  the  pro- 
duction of  disturbance  in  the  ether  when  a  molecule  propelled  from  the 
cathode  reaches  the  target,  and  accordingly  to  a  less  or  a  greater  thickness 
in  the  outward-travelling  shell  of  disturbance  in  the  ether;  and  that  at  rela- 
tiveh'  high  exhaustions  the  molecules  are  propelled  with  a  higher  velocity, 
and  so  give  rise  to  a  more  prompt  disturbance  when  they  reach  the  target. 

57 


MEMOIRS    ON 

Tliis  explains  tlie  absence,  or  almost  complete  absence,  of 
diffraction.  But  that  is  not  all  we  have  to  explain  ;  we  have 
still  a  very  serious  thing  behind.  What  is  it  that  constitutes 
the  difference  between  the  Rontgen  rays  and  rays  of  ordinary 
light  in  consequence  of  which  the  one  are  not  refracted,  or 
only  in  an  infinitesimal  degree,  while  the  other  are  freely  re- 
fracted ?  This  difficulty  led  me  to  conceive  of  a  theory, which 
I  believe  to  be  new,  as  to  the  nature  of  refraction  itself — as  to 
the  nature  of  what  takes  place,  for  example,  when  light  is  re- 
fracted through  a  prism.  Suppose  we  have  light  of  a  definite 
refrangibility,  and  a  prism  on  which  it  may  be  made  to  fall. 
When  the  light  is  admitted  we  commonly  imagine — at  least,  I 
believe  so — that  the  light  is  immediately  refracted,  and  with 
proper  appliances  you  get  the  spectrum.  Immediately  ?  I  do 
not  think  so.  How  is  it  that  light  travels  more  slowly  through 
refracting  medium  than  through  vacuum  ?  There  are  different 
conjectures  which  have  been  advanced.  One  is  that  the  ether 
within  refracting  media  is  more  dense  than  the  ether  in  free 
space.  Another  is  that  while  the  density  is  the  same  the  elas- 
ticity is  less.  Then,  there  have  been  speculations  as  to  the 
ether  being  loaded  with  particles  of  matter. 

Take  a  piano.  If  you  strike  a  note  a  string  is  set  in  vibration. 
You  would  hardly  hear  any  sound  at  all  if  it  were  rigidly  sup- 
ported. But  it  rests  on  a  bridge  communicating  with  a  sound- 
ing-board, and  the  sounding-board  presents  a  broad  surface  to 
the  air,  and  is  set  in  motion  by  the  string.  The  sounding- 
board  and  the  string  form  a  compound  vibrating  system.  In 
the  same  way  it  may  be  that  the  molecules  of  the  glass,  or  other 
refracting  medium,  and  the  ether  form  between  them  a  com- 
pound vibrating  system,  and,  ivhen  the  motion  is  fully  estab- 
lished, the  two  vibrate  harmoniously  together.  But  how  does 
it  get  to  be  established  ?  We  can  hardly  imagine  otherwise 
than  that  the  ether  is  excessively  rare  compared  with  ponder- 
able matter.*    Well,  supposing  the  ethereal  vibrations  start  and 

*  The  views  as  to  the  nature  of  refraction,  wliicii  I  have  endeavored  to 
explain,  lead  me  incidentally  to  make  a  remark  on  another  subject  not,  in- 
deed, very  closely  connected  with  it.    From  the  first,  ROntgen  recognized  as 

58 


ROXTGEX    RAYS 

reach  a  set  of  molecules,  they  are  somewhat  impeded  by  the 
molecules,  and  they  tend  also  to  move  the  molecules.  But  as 
the  molecules  are  relatively  very  heavy,  it  may  he  that  it  takes 

the  seat  of  the  X-rays  which  he  had  discovered  the  place  where  the  cathodic 
rays  fall  on  the  wall  of  the  Crookes'  tube.  This  place  is  indicated  to  the 
eye  by  the  fluorescence  of  the  glass.  But  we  are  not  on  that  account  to 
regard  the  fluorescence  as  the  cause  of  the  Rontgen  rays,  or  even  to  regard 
the  Rontgen  emission  as  a  sort  of  fluorescence.  I  have  seen  it  remarked, 
as  indicating  no  very  close  connection  between  the  two,  that  with  a  me- 
tallic target  we  have  a  copious  emission  of  Rontgen  raj-s  though  there  is  no 
fluorescence,  and  that  when  a  spot  on  the  ghiss  wall  of  a  Crookes'  tube  has 
for  some  time  been  exposed  to  a  rather  concentrated  cathodic  discharge, 
though  the  fluorescence  which  it  exhibits  under  the  action  of  the  cathodic 
discharge  becomes  comparatively  dull,  as  if  the  glass  were  in  some  way 
fatigued  for  fluorescence,  it  emits  the  Rontgen  rays  as  well  as  before. 

Fluorescence  is  undoubtedly  indicative  of  a  molecular  disturbance  ;  but 
in  what  precise  way  this  disturbance  is  brought  about  by  the  cathodic  dis- 
charge is  a  matter  on  which  I  refrain  from  speculating.  But  whatever  be 
the  precise  nature  of  the  process,  it  seems  pretty  evident  that  it  can  only 
be  by  repeated  impacts  of  molecules  from  the  cathode  that  a  sufficient 
molecular  disturbance  can  be  got  up  to  show  itself  as  a  visible  fluores- 
cence. 

Suppose  a  shower  of  molecules  from  the  cathode  to  be  allowed  suddenly 
to  fall  on  the  anti-cathode,  and  after  raining  on  it  for  a  little  to  be  as  sud- 
denly cut  off.  According  to  the  views  I  entertain  as  to  the  nature  of  the 
Rontgen  rays,  the  moment  the  shower  is  let  on  the  emission  of  Rontgen 
rays  begins,  it  lasts  as  long  as  the  shower,  and  ceases  the  moment  the  show- 
er is  cut  off.  But  the  fluorescence  onl}-  graduall3%  quickly  though  it  may 
be,  comes  on  when  the  shower  is  allowed  to  fall,  and  gradually  fades  awaj-- 
when  the  shower  is  cut  off.  So  far  from  the  fluorescence  being  in  any  way 
the  cause  of  the  Rontgen  emission,  there  seems  reason  to  think  that  if  it 
exercises  any  effect  upon  it  at  all,  it  is  rather  adverse  than  favorable.  For 
it  has  been  found  that  when  the  target  is  metallic,  and  gets  heated,  the 
Rontgen  discharge  falls  off ;  and  fluorescence,  like  a  rise  of  temperature, 
involves  a  molecular  disturbance,  though  the  kind  of  disturbance  is  differ- 
ent in  the  two  cases. 

As  the  fluorescence  of  the  glass  wall  and  the  emission  of  X-rays  are 
two  totally  different  effects  of  the  same  cause  —  namely,  the  molecular 
bombardment  from  the  cathode — the  intensity  of  the  one  must  by  no  means 
be  taken  as  a  measure  of  the  intensity  of  the  other,  even  with  the  same 
tube.  The  former  effect  would  appear  to  be  the  more  easily  produced. 
Tlds  consideration  removes  a  difficulty  mentioned  at  page  10  of  the  paper 

59 


MEMOIRS    OX 

some  considerable  time  for  the  molecules  to  be  set  sensibly  in 
motion.  Xow  if  the  system  of  molecules  is  exceedingly  com- 
plex, a  mode  of  motion  of  the  molecules,  or  it  may  be  of  the 
constituent  parts  of  the  molecules,  may  be  found  such  that 
the  system  tends  to  vibrate  in  practically  any  periodic  time 
that  you  may  choose ;  only  as  you  choose  one  time  or  another 
the  mode  of  vibration  will  be  different ;  and,  again,  according 
to  the  direction  in  which  the  molecules  are  successively  made 
to  vibrate  the  actual  mode  of  vibration  will  be  different.  Well, 
I  conceive  that  the  difference  between  the  propagation  of  the 
Runtgen  rays  and  rays  of  ordinary  light  with  reference  to  pass- 
ing through  a  prism  depends  upon  that.  When  you  let  a  ray 
of  light  fall  upon  a  refracting  medium  such  as  glass,  motions 
begin  to  take  place  in  the  molecules  forming  the  medium.  The 
motion  is  at  first  more  or  less  irregular  ;  but  the  vibrations 
ultimately  settle  down  into  a  system  of  such  a  kind  that  the 
regular  joint  vibrations  of  the  molecules  and  of  the  ether  are 
such  as  correspond  to  a  given  periodic  time,  namely,  that  of 
the  light  before  incidence  on  the  medium.  That  particular 
kind  of  vibration  among  the  molecules  is  kept  up,  while  the 
others  die  away,  so  that  after  a  prolonged  time — the  time  occu- 
pied by,  we  will  sky,  ten  thousand  vibrations,  which  is  only 
about  the  forty-thousand-millionth  part  of  a  second — the  mo- 
tion of  the  molecules  of  the  glass  has  gradually  got  up  until 
you  have  the  molecules  of  the  glass  and  the  ether  vibrating 
harmoniously  together.  But  in  the  case  of  the  Rontgen  rays, 
if  the  nature  of  them  be  what  I  have  explained,  you  have  a 
constant  succession  of  pulses  independent  of  one  another. 
Consequently  there  is  no  chance  to  get  up  harmony  between 
the  vibrations  of  the  ether  and  the  vibrations  of  the  body. 
Go  back  to  the  case  of  light  passing  through  glass.     When 

by  Prince  Galitzin  and  M.  v.  Karnojitzky,  as  attending  the  supposition 
that  the  X-rays  originate  in  the  points  in  which  the  cathodic  rays  fall  on 
tlie  wall  of  the  tube  or  other  target.  Nor  need  it  surprise  us  that  in  some 
cases  the  shadows  seem  to  indicate  more  than  one  source  of  action,  when 
we  remember  that  from  a  given  point  more  tbrin  one  normal  can  be  drawn 
to  a  given  closed  surface. 

60 


rOntgen  rays 

the  regular  combined  vibration  is  established  you  have  a  ki- 
netic energy,  due  partly  to  the  motion  of  the  ether  and  partly  to 
the  motion  of  the  molecules.  If  you  make  abstraction  of  the 
loss  of  energy  by  reflection,  the  rate  at  which  the  energy  passes 
within  the  glass  must  be  the  same  as  it  has  outside,  and  conse- 
quently there  must  be  the  same  energy  for  one  wave  length, 
which  corresponds  to  one  period  of  the  vibration,  inside  as  out- 
side. But  if  the  kinetic  energy  of  the  ether  is  the  same  for 
the  same  volume  inside  and  outside,  and  you  have  in  addition 
Inside  a  certain  amount  of  kinetic  energy  due  to  the  motion  of 
the  molecules,  the  two  taken  together  can  only  make  the  en- 
ergy for  a  wave  inside  the  same  as  for  a  wave  outside  on  the 
condition  that  the  velocity  of  propagation  inside  is  less  than 
the  velocity  of  propagation  outside.  That  is  the  theory  I  have 
been  forced  to  adopt  as  to  the  nature  of  refraction  in  conse- 
quence of  the  ideas  I  hold  as  to  the  nature  of  the  Rontgen 
rays  ;  and  if  you  adopt  that  theory  I  think  everything  falls  into 
its  place.  When  you  have  the  Rontgen  rays  falling  on  a  body, 
the  motion  of  the  ether  due  to  them  is  interfered  with  by  the 
molecules  of  the  body,  more  or  less.  Xo  body  is  perfectly 
transparent  to  these  rays,  and,  on  the  other  hand,  perhaps  we 
may  say  no  body  is  perfectly  opaque.  That  all  falls  into  its 
place  on  this  supposition  as  to  the  nature  of  the  action  of  the 
ether  on  the  molecules.  Xow,  why  is  it  that  the  Rontgen 
rays  do  not  care  whether  you  present  them  with  black  paper  or 
white  paper  ?  What  is  the  cause  of  blackness  ?  The  light 
falling  upon  the  paper  produces  motion  in  the  ultimate  mole- 
cules. In  the  case  of  a  transparent  substance  you  have  a  com- 
pound vibrating  system  going  on,  vibrating  without  change. 
But  in  the  case  of  an  absorbing  medium  the  vibrations  which 
after  a  time  are  produced  in  the  molecules  spread  out  into 
adjoining  molecules,  by  virtue  of  the  communication  of  the 
molecules  with  one  another,  and  are  carried  away  ;  so  that  in 
the  case  of  an  absorbing  medium  there  is  a  constant  beginning 
to  set  the  molecules  in  vibration  ;  but  they  never  get  to  the 
permanent  state,  because  the  vibration  is  carried  away  b}^  com- 
munication from  one  molecule  to  another.     But  in  the  case  of 

61 


MEMOIRS    OX 

the  Rontgen  rays  you  have  done  with  the  pulse  altogether 
long  before  any  harmonious  vibration  between  the  ether  and 
the  molecules  can  be  established  ;  so  that  a  state  of  things  is 
not  brought  about  in  which  you  get  a,  comparatively  speaking, 
large  vibration  of  the  molecules.  Consequently,  the  Rontgen 
rays  do  not  care  whether  you  give  them  black  paper  or  not. 

I  must  not  keep  you  more  than  a  minute  or  two  longer ;  but 
I  do  not  like  to  close  this  lecture  without  saying  a  word  or 
two  regarding  the  Becquerel  rays.  What  takes  place  there  ? 
To  be  brief,  I  must  refer  to  the  most  striking  case  of  all. 
Take  the  case  of  metallic  uranium.  That  gives  out  something 
which,  like  the  Rontgen  rays,  has  an  influence  passing  through 
black  paper,  and  capable  of  affecting  a  photographic  plate. 
It  is  also  capable  of  effecting  the  discharge  of  statically  charged 
electrified  bodies.  Apparently  this  goes  on  indefinitely.  You 
do  not  need,  apparently,  to  expose  the  metal  to  rays  of  high 
refrangibility  in  order  that  this  strange  thing  should  go  on. 
What  takes  place  ?  My  conjecture  is  that  the  molecule  of 
uranium  has  a  structure  which  may  be  roughly  compared  to  a 
flexible  chain  with  a  small  weight  at  the  end  of  it.  Suppose 
you  have  vibrations  communicated  to  such  a  chain  at  the  top ; 
they  travel  gradually  to  the  bottom,  and  near  the  bottom  pro- 
duce a  disturbance  w^iicli  deviates  more  from  a  simple  har- 
monic undulation.  So,  if  a  vibration  is  communicated  to 
what  I  win  call  the  tail  of  the  molecule  of  uranium,  it  may 
give  rise  to  a  disturbance  in  the  ether  which  is  not  of  a  regular 
periodic  character.  I  conceive,  then,  that  you  have  vibrations 
produced  in  the  ether,  not  of  such  a  permanently  regular  char- 
acter as  would  constitute  them  vibrations  of  light,  and  yet  not 
of  so  simple  a  character  as  in  the  Rontgen  rays — something  be- 
tween. And  accordingly  there  is  enough  irregularity  to  allow 
the  ethereal  disturbance  to  pass  through  black  i)aper,  and 
enough  regularity  on  the  other  hand  to  make  possible  a  cer- 
tain amount  of  refraction.  You  can  also  obtain  evidence  of 
the  polarization,  and,  consequently,  of  the  transverse  character 
of  these  rays. 

According  to  the  theory  of  the  nature  of  the  Runtgen  rays 

62 


RONTGEX    RAYS 

which  I  have  endeavored  very  briefly  to  bring  before  you,  we 
have  here,  as  I  think,  a  system  the  various  parts  of  which  fit  into 
one  another.  You  start  with  the  Rontgen  rays,  which  con- 
sist, as  I  conceive,  of  an  enormous  succession  of  independent 
pulses  ;  you  pass  to  the  Becquerel  rays,  which  are  still  irregu- 
lar, but  are  beginning  to  have  a  certain  amount  of  regularity  ; 
and  you  end  with  the  rays  which  constitute  ordinary  light. 
According  to  this  theory,  the  absence  of  diffraction  in  the 
Rontgen  rays  is  explained,  not  by  supposing  they  are  rays  of 
light  of  excessively  short  wave  length,  but  by  supposing  they 
are  due  to  an  irregular  repetition  of  isolated  and  independent 
disturbances.  So  far  as  I  know,  the  view  I  have  been  led  to 
form  as  to  the  nature  of  refraction,  and  which  forms  an  inte- 
gral portion  of  the  theory  as  to  the  Rontgen  rays,  is  altogether 
new,  so  much  so  that  I  felt  at  first  rather  startled  by  it ;  but 
I  found  myself  fairly  driven  to  it  by  the  ideas  I  entertain  as  to 
the  nature  of  the  Rontgen  rays,  and  I  am  not  aware  of  any  se- 
rious objection  to  it. 

Additional  Note 

The  problem  of  diffraction  in  the  case  of  a  vast  system  of 
inde])endent  very  slender  pulses  deserves  to  be  treated  in  some- 
what greater  detail.  It  is  rather  simpler  than  the  problem  of 
diffraction  in  the  case  of  series  of  undulations  such  as  those 
which  constitute  light,  because  the  pulses  are  to 'be  treated 
separately  and  independently,  like  streams  of  light  from  differ- 
ent sources ;  and  as  the  whole  thickness  of  a  pulse  in  the  case 
of  the, Rontgen  rays  may  probably  be  something  comparable 
with  the  millionth  of  an  inch,  we  have  no  need  to  inquire  what 
will  be  the  disturbance  continually  passing  across  a  fixed  sur- 
face in  space  ;  we  may  treat  the  shell  at  any  moment  as  consti- 
tuting an  initial  disturbance  in  the  ether,  and  then  examine  the 
efficiency  of  different  parts  of  the  shell  in  disturbing  at  a  future 
time  the  ether  at  a  given  point  of  space  in  front  of  the  shell. 

The  thickness  of  the  shell  is  not  necessarily  the  same  at 
points  situated  in  widely  different  directions  as  regards  their 
bearing  from  the  centre,  and  the  same  applies  to  the  direction 


MEMOIRS    ON 


of  disturbance.  But  in  any  case  for  a  small  portion  of  the 
shell  the  thickness  may  be  deemed  uniform,  and  the  direction 
of  disturbance  sensibly  the  same  as  we  pass  from  point  to  point 
in  a  direction  tangential  to  the  shell,  while  it  varies  with  great 
rapidity,  at  least  as  regards  its  amount,  when  we  pass  from 
point  to  point  in  a  normal  direction,  vanishing  at  the  outer  and 
inner  boundaries  of  the  shell. 

As  the  disturbance  we  are  concerned  with  is  of  the  distor- 

tional  kind  only,  the  disturbance 
at  time  t  at  a  point  P  in  front 
of  the  shell  may  be  obtained 
from  that  at  time  0  in  the  shell 
in  its  position  wdiich  is  taken  as 
initial  by  the  last  equation  in  Art. 
'Z'l  of  my  paper  on  diffraction  al- 
ready cited.  Let  E  be  a  point  in 
the  shell  of  disturbance  when  in 
that  position  which  is  regarded 
as  initial,  r,  r'  the  distances  PR, 
OR ;  d,  6'  their  inclinations  to 
OP ;  (p  the  azimuth  round  OP 
of  the  plane  PRO.  Then  in  the 
formula  referred  to  da  =  sin  ddddcp.  Also  rddxs'm  {6-^d')=dr'; 
and  sin  61/sin  {6  +  e')-r'IOP  =  r'/(r  +  r')  very  nearly. 

Let  OP  cut  the  inner  boundary  of  the  shell  in  S,  and  let 
ab  or  QS,  the  thickness  of  the  shell,  be  denoted  by  X.  In  the 
equation  referred  to,  the  term  arising  from  the  differentiation 
with  respect  to  t  of  the  t  outside  the  sign  of  double  integration 
will  be  of  the  order  X/r'  as- compared  with  the  others,  and  may, 
therefore,  be  neglected.  Tlie  t  outside  may  be  replaced  by 
r/b,  and  the  fraction  r/(r-\-r'),  being  sensibly  constant  over 
the  range  of  integration,  may  be  put  outside.  Our  expression 
then  becomes 


-^^=Fi7//("-^S):r 


/.p. 


*  Tlie  suffix  bt  means  that  tliu  intcgraliou  is  taken  over  a  spherical  sur- 
face with  centre  Pand  radius  bt. 

64 


RONTGEN  RAYS 

As  the  disturbance  deemed  initial  was  only  a  momentary  con- 
dition of  a  wave  that  had  been  travelling  outwards  with  the 

velocity  h,  we  must  have  iiQ=--h-—^^,  and  therefore 

dr' 


'^'—^Jf&^''^^- 


The  expression  is  left  in  the  first  instance  in  this  shape  in 
order  to  show  more  clearly  tlie  manner  in  which  each  portion  of 
the  disturbance  in  the  state  taken  as  initial  contributes  towards 
the  future  disturbance  at  F.     When  there  is  no  obstacle  to  the 

transmission  we  shall  have    j  dcp—^ir,  and    /  ( -7-7)^^^''=  (^o) 

taken  between  limits.  If  It  <PQ,  the  sphere  round  P  with 
radius  M  does  not  cut  the  disturbed  region  at  all,  and  the 
disturbance  at  P  is  nil.  If  bt^PS,  the  limits  of  /  are  the 
distances  from  0  at  which  the  sphere  round  P  cuts  the  inner 
and  outer  limits  of  the  shell,  and  as  the  disturbance  there  van- 
ishes we  have  again  no  disturbance  at  P.  But  if  M  lies  be- 
tween those  limits,  and  the  sphere  round  P  cuts  OP  in  T 
(which  point  must  lie  between  Q  and  8)  the  limits  of  r'  will  be 
OT  io  a  point  in  the  outer  boundary  of  the  shell,  where  there- 
fore ^0  vanishes.  Hence  the  displacement  at  P  is  the  same  as 
was  initially  at  T,  only  diminished  in  the  ratio  of  r-\-r'  to  '/, 
as  we  know  it  ought  to  be. 

Reverting  to  the  expression  for  I  given  by  the  double  inte- 
gral, we  see  that  the  only  portion  of  the  shell  which  is  efficient 
in  producing  a  subsequent  disturbance  at  P  lies  between  the 
sphere  round  0  with  radius  OQ  and  the  sphere  round  P  with 
radius  Pas'.  If  /3  be  the  distance  from  OP  of  the  intersection 
of  these  spheres,  we  have,  considering  the  smallness  of  the  ob- 
liquities. 

If  we  suppose  r  and  r'  to  be  each  4  inches,  and  \  the  mill- 
ionth of  an  inch,  we  have  /3  =  0.002  inch,  so  that  at  a  distance 
not  less  than  the  one-250th  of  an  inch  from  the  projection  of 
E  65 


MEMOIRS  ON  RGNTGEX  RAYS 

the  edge  of  an  opaque  body  intercepting  Eontgen  rays  coming 
from  a  point  4  inches  off,  and  received  on  a  screen  (fluorescent 
or  photographic)  4  inches  on  the  other  side,  there  would  be 
full  effect  or  no  effect  according  as  we  take  the  illuminated  or 
the  dark  side  of  the  projection.  We  see  then  how  possible  it 
may  be  to  have  an  almost  complete  absence  of  diffraction  of 
the  Rontgen  rays  if  the  pulses  are  as  thin  as  above  supposed  ; 
and  as  these  rays  are  started  in  the  first  instance  in  a  totally 
different  manner  from  rays  of  ordinary  light,  namely,  by  the 
arrival  of  charged  molecules  from  a  cathode  at  a  target  instead 
of  by  the  vibrations  of  the  molecules  of  ponderable  matter,  we 
know  of  no  reason  beforehand  forbidding  us  to  attribute  an 
excessive  thinness  to  the  pulses  which  the  charged  molecules 
excite  in  the  ether. 


Biographical  Sketch 

Sir  George  Gabriel  Stokes  was  born  August  13,  1819,  in 
Ireland,  County  Sligo,  and  is  at  the  present  time  Fellow  of 
Pembroke  College  and  Lucasian  Professor  of  Mathematics  in 
the  University  of  Cambridge.  He  was  Senior  Wrangler  in 
1841  ;  he  has  been  President  of  the  Royal  Society  of  London, 
and  has  received  numerous  honors  at  home  and  abroad.  His 
main  contributions  to  science  may  be  grouped  under  the  head 
of  Hydrodynamics  and  the  Wave  Theory  of  Light.  His  papers 
on  the  former  subject  gave  the  first  rigid  treatment  and  formed 
the  basis  of  our  modern  theory.  Similarly,  in  the  wave  theory 
of  light  his  papers  on  the  dynamical  theory  of  diffraction  and 
on  the  aberration  of  light  are  two  of  the  most  important  con- 
tributions of  modern  times  to  science.  His  collected  papers 
are  being  published  at  the  present  time  by  the  University  of 
Cambridge,  and  two  volumes  have  already  appeared. 

His  experimental  work  has  been  largely  in  connection  with 
such  optical  phenomena  as  fluorescence,  metallic  reflection, 
and  certain  anomalous  colors  seen  in  crystals. 

His  mathematical  work  is  of  the  first  importance,  and  his 
numerous  contributions  to  all  branches  of  mathematical  physics 
have  been  of  the  greatest  service  to  science. 

66 


A    THEORY    OF    THE    CONNECTION    BE- 
TWEEN CATHODE  AND  RONTGEN  RAYS 

BY 

J.  J.  THOMSON,  M.A.,  F.R.S., 

Cavendish  Professor  of  Experimental  Phj'sics,  Cambridge 
(^Philosophical  Magazine,  February,  1898) 


CONTEXTS 

PAGE 

ElietrijSed  Bxrtide  JM&tiiuf  in  Magrndie  Fidd, 69 

Mntffen,  Eitjf*,  thin  PnU^ 70 

Veltmty  qf  Mmtiffen  and  Lmard  Mxiftt. 71 

Pul»^  Gon^iM  <sf  Eleetrk  and  MMgntftic  Di^uHkinoe* 71 

Efeet  of  Time  «f  ChUintfn, 71 

0»mmunimti*!fn  of  Energy  to  Charsf&i  lom 73 

DiMwrianee  Greated  at  Right  Angk*  to  the  Cathode  Ba§« 72 

Mntgen  Bajf*  mot  Wanes,  but  Imput*e*. 73 


A    THEORY     OF    THE    COXXECTIOX    BE- 
TWEEN CATHODE  AXD  ROXTGEX  RAYS 

BY 

J.  J.  THOMSO\,.M.A.,  F.R.S. 

A  MOTIXG  electrified  particle  is  surronnded  by  a  magnetic 
field,  the  lines  of  magnetic  force  being  circles  having  the  line 
of  motion  of  the  particle  for  axis.  If  the  particle  be  sud- 
denly stopped,  there  will,  in  consequence  of  electro-magnetic 
induction,  be  no  instantaneous  chancre  in  the  ma^etic  field  ; 
the  induction  gives  rise  to  a  magnetic  field  which  for  a  mo- 
ment compensates  for  that  destroyed  by  the  stopping  of  the 
particle.  The  new  field  thus  introduced  is  not,  however,  in 
equilibrium,  but  moves  off  through  the  dielectric  as  an  electric 
pulse.  In  this  paper  we  calculate  the  magnetic  force  and 
electric  intensity  carried  by  the  pulse  to  any  point  in  the  di- 
electric. 

The  distribution  of  magnetic  force  and  electric  intensity 
around  the  moving  particle  depends  greatly  on  the  velocity 
of  the  particle :  if  this  velocity  is  so  smaU  that  the  square 
of  its  ratio  to  the  velocity  of  light  can  be  neglected, 
then  the  electric  intensity  is  symmetrically  distributed  round 
the  particle,  and  at  a  distance  r  from  it  is  equal  to  ejr^, 
where  e  is  the  charge  on  the  particle  ;  the  lines  of  magnetic 
force  are  circles  with  the  line  of  motion  of  the  particle  for 
axis  :  the  magnitude  of  the  magnetic  force  at  a  point  P  is 
we  sin  Q/r^,  where  ic  is  the  velocity  of  the  particle,  and  0  the 
angle  a  radius  from  the  particle  to  P  makes  with  the  direc- 
tion of  motion. 

When,  however,  the  velocity  of  the  particle  is  so  great  that 

69 


MEMOIRS    ON 

we  can  no  longer  neglect  the  square  of  its  ratio  to  the  velocity 
of  light,  the  distribution  of  electric  intensity  is  no  longer  uni- 
form ;  the  electric  intensity,  along  with  the  magnetic  force, 
tends  to  concentrate  in  the  equatorial  plane — that  is,  the  plane 
through  the  centre  of  the  particle  at  right  angles  to  its  direc- 
tion of  motion  ;  this  tendency  increases  with  the  velocity  of 
the  particle  until,  when  this  is  equal  to  the  velocity  of  light, 
both  the  magnetic  force  and  the  electric  intensity  vanish  at  all 
parts  of  the  field,  except  the  equatorial  plane,  and  in  this  plane 
they  are  infinite. 

The  pulses  started  by  the  stopping  of  the  charged  particle 
are,  as  might  be  expected,  different  when  the  ratio  of  the  ve- 
locity of  the  particle  to  that  of  light  is  small  and  when  it  is 
nearly  unity.  But  even  when  the  velocity  is  small,  the  pulse, 
started  by  stopping  the  j^article,  carries  to  an  external  point  a 
disturbance  in  which  the  magnetic  force  is  enormously  greater 
than  it  was  at  the  same  point  before  the  particle  was  stopped. 
The  time  the  pulse  takes  to  pass  over  apoint  P  is,  if  the  charged 
particle  be  spherical,  equal  to  the  time  light  takes  to  pass  over 
a  distance  equal  to  the  diameter  of  this  sphere  ;  the  thickness 
of  this  pulse  is  excessively  small  compared  with  the  wave-length 
of  visible  light.  When  the  velocity  of  the  particle  approaches 
that  of  light,  two  pulses  are  started  when  it  is  stopped.  One  of 
these  is  a  thin  plane  sheet  whose  thickness  is  equal  to  the  di- 
ameter of  the  charged  particle  ;  this  wave  is  propagated  in  the 
direction  in  which  the  particle  was  moving ;  there  is  no  corre- 
sponding wave  propagated  backwards :  the  other  is  a  spherical 
pulse,  spreading  outward  in  all  directions,  whose  thickness  is 
again  equal  to  the  diameter  of  the  charged  particle,  and  thus, 
if  the  particle  is  of  molecular  dimensions,  or,  perhaps,  even 
smaller,  very  small  compared  with  the  wave-length  of  ordinary 
light.  The  theory  I  wish  to  put  forward  is  that  the  Rontgen 
rays  are  these  thin  pulses  of  electric  and  magnetic  disturbances 
which  are  started  when  the  small  negatively  charged  particles 
which  constitute  the  cathode  rays  are  stopped. 

[^The  matliematical  theory  is  omitted.^ 

Thus  we  see  that  the  stoppage  of  a  charged  particle  will  give 

70 


ROXTGEX    RAYS 

rise  to  very  thin  pulses  of  intense  magnetic  force  and  electric 
intensity ;  when  the  velocity  of  the  particle  is  small  there  will 
be  one  spherical  pnlse  ;  when  the  velocity  is  nearly  equal  to 
that  of  light  there  will,  in  addition  to  the  spherical  pulse,  be  a 
plane  one,  propagated  only  in  the  direction  in  which  the  par- 
ticle was  originally  moving.  It  is  these  pulses  which  I  believe 
constitute  the  Rontgen  rays.  As  they  consist  of  electric  and 
magnetic  disturbances,  they  might  be  expected  to  produce  some 
effects  analogous  to  those  of  light.  If  they  were  so  thin  that 
the  time  taken  by  them  to  pass  over  a  molecule  of  a  substance 
were  small  compared  with  the  time  of  vibration  of  the  mole- 
cule, there  would  be  no  refraction,  and  the  thinness  of  the  pulse 
would  also  account  for  the  absence  of  diffraction. 

In  the  preceding  investigation  we  have  supposed  that  the 
stoppage  of  the  particle  is  instantaneous ;  if  the  impact  lasts  for 
a  finite  time  T,  the  negative  pulse  will  be  broadened  out,  so 
that  its  thickness,  instead  of  being  2a,  will  be  2a-\-\T,  where 
V  is  the  velocity  of  light.  The  intensity  of  the  magnetic 
force  in  the  pulse  will  vary  inversely  as  the  thickness  of  the 
pulse,  so  that,  when  the  collision  lasts  for  the  time  T,  the  mag- 
netic force  in  the  negative  pulse  will  be  2a}{2a-\-\T)  of  the 
value  given  above.  The  more  sudden  the  collision,  the  thinner 
the  pulse  and  the  greater  the  magnetic  force  and  the- energy  in 
the  pulse ;  the  pulse  will,  however,  possess  the  properties  of 
the  Rontgen  rays  until  T  is  comparable  to  one  of  the  times  of 
vibration  of  a  substance  through  which  it  has  to  pass.  In  the 
case  of  the  cathode  rays  all  the  circumstances  seem  favorable 
to  a  very  sudden  collision,  as  the  mass  of  the  moving  particles 
is  very  small  and  their  velocity  exceedingly  great.  In  some  ex- 
periments which  I  described  in  the  Philosophical  Ilagazine  for 
October,  1897,  on  cathode  rays,  the  velocity  of  the  negative 
particles  was  about  one-third  of  that  of  light,  and  in  some  more 
recent  experiments  made  on  the  Lenard  rays,  with  the  appa- 
ratus described  by  Des  Coudres,  considerably  higher  velocities 
were  found.  A  change  in  the  time  of  the  collision  will  alter 
the  thickness  of  the  pulse,  and  so  change  the  nature  of  the 
ray. 

71 


MEMOIRS    OX 

If  we  suppose  that  part  of  the  absorption  of  the  rays  is  clue 
to  the  communication  of  energy  to  charged  ions  in  their  path, 
we  find  that  the  thicker  the  pulse  the  greater  the  absorption. 
For,  suppose  that  E  is  the  electric  intensity  in  the  pulse,  m  the 
mass,  and  e  the  charge  on  an  ion ;  then,  if  u  is  the  velocity  com- 
municated to  the  ion  when  the  pulse  passes  over  it,  t  the  time 
taken  by  the  pulse  to  pass  over  it, 

7nu='Ee .  t ; 
or,  if  cl  is  the  thickness  of  the  pulse, 

V 

thus  the  energy  -imc^  communicated  to  the  ion  is  equal  to 


1  E\ri 


2      \' 

Xow  the  energy  in  the  pulse  is  proportional  to  E'^  d/Y^,  so 
that  the  ratio  of  the  energy  communicated  to  the  ion  to  the 
energy  in  the  pulse  is  proportional  to  d.  Thus,  the  broader 
the  pulse,  the  greater  the  absorption  and  the  less  the  penetrat- 
ing power.  The  energy  in  the  pulse  is  inversely  proportional 
to  its  thickness. 

If  we  return  to  the  expression  for  the  intensity  of  the  mag- 
netic force  in  case  (1),  we  see  that  it  is  proportional  to  sin  0, 
so  that  the  disturbance  is  greatest  at  right  angles  to  the 
cathode  rays  :  thus,  if  the  cathode  particles  are  stopped  at  their 
first  encounter,  the  Rontgen  rays  would  be  brightest  at  right 
angles  to  the  cathode  rays  ;  if,  however,  as  would  seem  most 
probable,  the  cathode  particles  had  to  make  several  encounters 
before  they  were  reduced  to  rest,  changing  their  direction  be- 
tween each  encounter,  the  distribution  of  the  cathode  rays 
would  be  much  more  uniform.  Experiments  on  the  distribu- 
tion of  Rontgen  rays  produced  by  the  impact  of  the  cathode 
particles  directly  against  the  walls  of  the  discharge-tube  are,  as 
Sir  George  Stokes  has  pointed  out,  affected  by  the  much  greater 
absorption  of  the  oblique  rays  produced  by  the  greater  thick- 
ness of  glass  traversed  by  them.  Experiments  on  rays  produced 
by  focus-tubes  would  give  results  more  easily  interpreted. 

73 


RONTGEX    RAYS 

The  result  to  which  we  have  been  led  from  the  consideration 
of  the  effects  produced  by  the  sudden  stoppage  of  an  electrified 
particle,  viz.,  that  the  Rontgen  effects  are  produced  by  a  very 
thin  pulse  of  intense  electro-magnetic  disturbance,  is  in  agree- 
ment with  the  view  expressed  by  Sir  George  Stokes  in  the 
Wilde  Lecture  (^^Proceedings  of  the  Manchester  Literary  and 
Philosophical  Society/'  1897),  that  the  Rontgen  rays  are  not 
waves  of  very  short  wave-length,  but  impulses. 

Cambridse,  December  16,  1897. 


Biographical  Sketch 

Joseph  Johx  Thomson  was  born  Dec.  18, 1856,  in  Manches- 
ter, and  is  at  the  present  time  Fellow  of  Trinity  College  and 
the  Cavendish  Professor  of  Experimental  Physics  at  Cambridge. 
Professor  Thomson  was  second  Wrangler  in  1880,  and  was  ap- 
pointed to  his  professorship  in  1881:.  He  has  contributed 
greatly  to  our  knowledge  of  both  practical  and  theoretical 
physics ;  his  researches  on  the  theory  of  vortices,  and  on  the 
application  of  dynamics  to  physical  problems,  have  been  pub- 
lished in  book  form.  The  greatest  debt  that  science  owes 
him,  however,  is  for  having  introduced  system  and  order  into 
the  vast  collection  of  experimental  data  which  have  been  ac- 
cumulated concerning  the  discharge  of  electricity  through 
gases.  His  work  on  this  subject  has  been  carried  on  during 
the  past  ten  years,  and  the  most  important  conclusions  are  con- 
tained in  his  volume  Recent  Researches  in  Electricity  and  Mag- 
netism, Oxford,  1893.  Professor  Thomson  has  contributed 
largely  to  our  present  knowledge  of  cathode  rays  and  to  all 
that  pertains  to  the  connection  between  matter  and  electricity, 
particularly  to  the  explanation  of  electrolysis  and  ionization. 

73 


BIBLIOGRAPHY 

Among  the  most  important  papers  bearing  upon  the  subject  of  X-ra}?; 
are  the  following  : 

Cathode  Rays : 

Lenard,  Wiedemann,  Annalen,  51,  1894  ;  56,  1895  ;  63, 1897. 
Thomson,  J.  J.,  Philosophical  Magazine,  38,  1894  ;  44, 1897. 

British  Association  Report,  1896. 
McClelland,  Proceedings  Royal  Society  of  London,  61,  1897. 
Perriu,  Comptes  Rendus,  121, 1895. 

RoxTGEN  Rays  : 

Thomson,  J.  J.,  Proceedings  Royal  Society  of  London,  59,  1896. 

Nature,  53,  54,  ^5,  58,  1896-1898. 
Perrin,  Annales  de  Chimie  et  de  Physique,  11, 1897. 

Comptes  Rendus,  122,  123,  124,  126,  1896-1898. 
Murray,  J.  R.  E.,  Proceedings  Royal  Society  of  London,  59,  1896. 
Wilson,  C.  T.  R.,  Proceedings  Royal  Society  of  London,  59, 1896. 
Lehmann,  Zeitschrift  fUr  Electrochemie,  1, 1896. 
Lord  Rayleigh,  Nature,  57,  1898. 
Les  Rayons  Xet  la  Photographic  a  Travel's  les  Corps  Opaques.     Par  Cli. 

Ed.  Guillaume,  Paris,  1896. 
Rontgen  Rays  and  Phenomena  of  the  Anode  and  Cathode.     By  E.  P. 
Thompson,  New  York,  1896. 

Becquerel  Rays  : 

Becquercl,  H.,  Comptes  Rendus,  122,  123,  124, 1896-1897. 
Sagnac,  G.,  Journal  de  Physique,  5, 1896. 

A  Resume  of  the  Experiments  dealing  with  the  Properties  of  Becquerel 
Rays.     By  O.  M.  Stewart,  Physical  Review,  6,  April,  1898. 

Thorium  Rays  : 

Schmidt,  G.  C,  Wiedemann,  Annalen,  65, 1898. 
Sklodowska-Curie,  Comptes  Rendus,  126,1898. 


INDEX 


Absorption  of  Rays  bv  Charged 
Ions,  72. 

Air,  Active  when  Exposed  to  X-rays, 
15,  21,  22  ;  How  it  Loses  Activi'ty, 
16  ;  ^lore  Transparent  to  X-rays 
than  to  Cathode  Rays,  10. 

Aluminium,  Transparency  of,  4.  1-3, 
27,  30,  52. 

B 

Barium  Platino- Cyanide,    6,   23; 

Screen  of.  3,  21,  22.  23. 
Becquerel  Rays,  48,  62,  63. 
Bibliography,  74. 
Blackness,  Cause  of,  61. 
Brandes's  Experiment,  39,  40. 
Brightness,  Conditions  of,  24. 


Calcium  Tungstate,  23. 

Cathode  Ravs,  10.  11.  24.  35,  38,  39, 
70,  72  ;  Speed  of,  71  ;  Relation  to 
Rontgen  Ravs,  70. 

Cathodic  Rays,  43,  44,  49,  50,  52, 
53  ;  Stream's  of  Electrified  Mole- 
cules, 53. 

Crookes,  W.,  Experiments  of,  50. 

Current,  Effective,  in  Tubes,  34. 

D 

Density,  as  Determining  Transpa- 
rency. 5. 

Diffraction  Phenomena.  40,  46,  53, 
55,  56,  57,  63,  64,  65,  71. 

Direction,  Variation  of  Intensity 
with,  24. 


Edison  Screen,  23. 


Electrical  Intensity  and  Magnetic 
Force,  Carried  by  Pulse,  69." 

Electrified  Bodies,  Discharged  by  X- 
rays,  13. 

Energy  in  Pulse,  Inversely  Propor- 
tional to  Thickness,  72. 

Eye,  Sensitiveness  to  X-rays,  7,  39. 


Fluorescence,  Production  of,  by  X- 
rays,  3,  6,  36,  59. 

G 

Galitzin,  Prince,  44,  46,  47.  60. 
Glass.  Transparencv  of,  to  X-rays, 

4,  17. 
Green,  George,  48. 

H 
Hertz,  Heinrich,  10,  35. 

I 
Interference,  12. 
Interrupters  of  Deprez  and  Foucault, 

31,  33. 
Ions,  Absorption  of  Rays  by  Charged, 


Karnojitzky,  von,  M..  44,  46,  47,  60. 
Kelvin.  Lord,  on  Static  Charge,  53. 
Kinetic  Energy  of  Eiher,  61. 


Lambert's  Law,  25. 

Lenard  Ra\'s,  Nature  of.  52  :  Speed 

of,  71. 
Lenard "s  Experiments,  10,  36,  49,  50, 

51.  52. 
Longitudinal  Vibrations  in  the  Ether, 

13,  47. 


75 


INDEX 


M 
Magnetic  Force,  Carried  by  Pulse, 

69. 
McClelland,  C.  M.,  45. 
Molecules,  Statically  Charged,  53. 

N 
Normal  Rays,  More  Copiously  Emit- 
ted, 45. 


Photographic  Action,  6  ;  and  Fluo- 
rescent Action  Different,  37. 

Photometer  for  X  rays,  23  ;  Weber, 
10;  Bouguer,  23. 

Platinum- Aluminium  Window,  29  ; 
Transparency  of,  to  X-rays.  4,  17. 

Polarization  of  X-rays  not  Proved, 
47. 

Powders,  Effect  of,  8. 

Pulses,  53,  54,  72  ;  by  Impact  of 
Charged  Particles,  69,  70,  71  ; 
Vast  System  of,  Mathematically 
Treated,  63,  65  ;  Velocity  of,  70. 

R 

Radiation,  Intensity  of,  23,  24. 

"  Raj''s"  Defined,  11. 

Reflection  not  Observed  with  X- 
rays,  8,  46. 

Refraction,  Nature  of,  58,  59  ;  New 
Theory  of,  58,  63  ;  Not  Observed 
with  X-rays,  7,  46,  71. 

Rontgen,  W.  L.,  Biographical  Notice 
of,  40. 

Rontgen  Rays,  a  Vast  Succession  of 
Independent  Pulses,  54;  Difference 
of  Quality  in,  56,  57  ;  Thin  Pulses, 
Electric  and  Magnetic  Disturb- 
ances, 70. 


Shadows,  Sharpness  of,  11,  44,  46, 
57  ;   Double,  44. 

Spark -Gap.  32. 

Stokes,  G.  G.,  26,  43,  73  ;  Biographi- 
cal Notice  of,  66. 


Tesla  Apparatus,  17,  13,  32. 

Thickness,  P2ffect  of,  on  Transpa- 
rency, 5;  Relation  of,  to  Density,  6. 

Thomson,  J.  J.,  45  ;  Biographical 
Notice  of,  73. 

Transparency  of  Different  Bodies, 
4,  26,  27,  46. 

Tubes,  Hard  and  Soft,  30,  33  ;  Hit- 
torf,  Lenard,  Crookes,  3,  9,  10, 
35,  39,  50  ;  Variation  of,  with 
Use,  33. 


Velocity,  Dependence  of  Pulse  on, 

70. 
Vibration,     Longitudinal,     in      the 

Ether,  13,  47. 

W 

Weber  Photometer,  10. 
Wiedemann,  E.,  52. 
Wilde  Lecture,  43. 
Window-Number,  29. 

X 

X-rays,  Best  Tubes  for,  24  ;  Chemi- 
cal Action  of,  6  ;  Difference  with 
Hard  and  Sofi  Tul)e«;,  31  ;  Diffrac- 
tion of,  40 ;  Discharge  Electrified 
Bodies,  13  ;  Discovery  of.  3  ;  Dis- 
tinction from  Cathode  Rays,  11  ; 
Effect  of  Current  on,  34 ;  Intensity 
of,  23  ;  Interference  of,  12  :  Mixt- 
ure of,  35  ;  Named,  4  ;  Nature 
of,  12,  43  ;  Not  Deflected  by  Mag- 
net, 10  ;  Origin  in  Tube,  11.  38, 
43  ;  Photograph  by  jNIeans  of.  6  ; 
Photometer,  23 ;  JPlatinum  Best 
Suited  to,  18  ;  Propagation  in  the 
Ether,  46 ;  Rectilinear  Propaga- 
tion of,  11,  12  ;  Succession  of 
Pulses,  54  ;  Transparency  to.  3, 
4,  10,  26.  27,  28;  Transverse  Vi- 
bration of,  48. 

Z 

Zehnder  Tubes,  34. 


THE    END 


STANDARDS  IN  NATURAL  SCIENCE 


COMPARATIVE     ZOOLOGY 

Structural  and  Systematic.  For  use  in  Schools  and  Col- 
leges. By  James  Ortojs",  Ph.D.  New  edition,  revised 
by  Charles  Wright  Dodge,  M.S.,  Professor  of  Biology 
in  the  University  of  Koch  ester.  With  350  illustrations. 
Crown  8vo,  Cloth,  SI  80  ;  by  mail,  II  96. 

The  distinctive  character  of  this  work  consists  in  the  treatment  of  the 
whole  Animal  Kingdom  as  a  unit ;  in  the  comparative  study  of  the  devel- 
opment and  variations  of  organs  and  their  functions,  from  the  simplest  to 
the  most  complex  state;  in  withholding  Systematic  Zoology  until  the 
student  has  mastered  those  structural  affinities  upon  which  "true  classifi- 
cation is  founded  ;  and  in  being  fitted  for  High  Schools  and  Mixed  Schools 
by  its  language  and  illustrations,  yet  going  far  enough  to  constitute  a  com- 
plete grammar  of  the  science  for  the  undergraduate  course  of  any  college. 

INTRODUCTION     TO     ELEMENTARY     PRACTICAL 
BIOLOGY 

A  Laboratory  Guide  for  High  Schools  and  College  Students. 

By  Charles  Wright  Dodge,  M.S.,  Professor  of  Biology, 

University  of   Eochester.     Crown   8vo,   Cloth,  $1  80 ;    by 

mail,  81  95. 

Professor  Dodge's  manual  consists  essentially  of  questions  on  the  struct- 
ure and  the  physiology  of  a  series  of  common  animals  and  plants  typical 
of  their  kind — questions  which  can  be  answered  only  by  actual  examination 
of  the  specimen  or  by  experiment.  Directions  are  given  for  the  collection 
of  specimens,  for  their  preservation,  and  for  preparing  them  for  examina- 
tion ;  also  for  performing  simple  physiological  experiments.  Particular 
species  are  not  required,  as  the  questions  usually  apply  well  to  several 
related  forms. 

THE    STUDENTS'    LYELL 

A  Manual  of  Elementary  Geology.  Edited  by  JoHiS'  W. 
JuDD,  C.B.,  LL.D.,  F.R.S.,  Professor  of  Geology,  and  Dean 
of  the  Royal  College  of  Science,  London.  With  a  Geologi- 
cal Map,  and  736  Illustrations  in  the  Text.  New,  revised 
edition.     Crown  8vo,  Cloth,  $2  26 ;  by  mail,  12  39. 

The  progress  of  geological  science  during  the  last  quarter  of  a  century 
has  rendered  necessary  very  considerable  additions  and  corrections,  and 
the  rewriting  of  large  portions  of  the  book,  but  I  have  everywhere  striven 
to  preserve  the  author's  plan  and  to  follow  the  methods  which  characterize 
the  original  work. — Extract  from  the  Preface  of  the  Jtievised  Edition. 


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